| |
| .. _expressions: |
| |
| *********** |
| Expressions |
| *********** |
| |
| .. index:: expression, BNF |
| |
| This chapter explains the meaning of the elements of expressions in Python. |
| |
| **Syntax Notes:** In this and the following chapters, extended BNF notation will |
| be used to describe syntax, not lexical analysis. When (one alternative of) a |
| syntax rule has the form |
| |
| .. productionlist:: * |
| name: `othername` |
| |
| and no semantics are given, the semantics of this form of ``name`` are the same |
| as for ``othername``. |
| |
| |
| .. _conversions: |
| |
| Arithmetic conversions |
| ====================== |
| |
| .. index:: pair: arithmetic; conversion |
| |
| When a description of an arithmetic operator below uses the phrase "the numeric |
| arguments are converted to a common type," this means that the operator |
| implementation for built-in types works as follows: |
| |
| * If either argument is a complex number, the other is converted to complex; |
| |
| * otherwise, if either argument is a floating point number, the other is |
| converted to floating point; |
| |
| * otherwise, both must be integers and no conversion is necessary. |
| |
| Some additional rules apply for certain operators (e.g., a string as a left |
| argument to the '%' operator). Extensions must define their own conversion |
| behavior. |
| |
| |
| .. _atoms: |
| |
| Atoms |
| ===== |
| |
| .. index:: atom |
| |
| Atoms are the most basic elements of expressions. The simplest atoms are |
| identifiers or literals. Forms enclosed in parentheses, brackets or braces are |
| also categorized syntactically as atoms. The syntax for atoms is: |
| |
| .. productionlist:: |
| atom: `identifier` | `literal` | `enclosure` |
| enclosure: `parenth_form` | `list_display` | `dict_display` | `set_display` |
| : | `generator_expression` | `yield_atom` |
| |
| |
| .. _atom-identifiers: |
| |
| Identifiers (Names) |
| ------------------- |
| |
| .. index:: name, identifier |
| |
| An identifier occurring as an atom is a name. See section :ref:`identifiers` |
| for lexical definition and section :ref:`naming` for documentation of naming and |
| binding. |
| |
| .. index:: exception: NameError |
| |
| When the name is bound to an object, evaluation of the atom yields that object. |
| When a name is not bound, an attempt to evaluate it raises a :exc:`NameError` |
| exception. |
| |
| .. index:: |
| pair: name; mangling |
| pair: private; names |
| |
| **Private name mangling:** When an identifier that textually occurs in a class |
| definition begins with two or more underscore characters and does not end in two |
| or more underscores, it is considered a :dfn:`private name` of that class. |
| Private names are transformed to a longer form before code is generated for |
| them. The transformation inserts the class name, with leading underscores |
| removed and a single underscore inserted, in front of the name. For example, |
| the identifier ``__spam`` occurring in a class named ``Ham`` will be transformed |
| to ``_Ham__spam``. This transformation is independent of the syntactical |
| context in which the identifier is used. If the transformed name is extremely |
| long (longer than 255 characters), implementation defined truncation may happen. |
| If the class name consists only of underscores, no transformation is done. |
| |
| |
| .. _atom-literals: |
| |
| Literals |
| -------- |
| |
| .. index:: single: literal |
| |
| Python supports string and bytes literals and various numeric literals: |
| |
| .. productionlist:: |
| literal: `stringliteral` | `bytesliteral` |
| : | `integer` | `floatnumber` | `imagnumber` |
| |
| Evaluation of a literal yields an object of the given type (string, bytes, |
| integer, floating point number, complex number) with the given value. The value |
| may be approximated in the case of floating point and imaginary (complex) |
| literals. See section :ref:`literals` for details. |
| |
| .. index:: |
| triple: immutable; data; type |
| pair: immutable; object |
| |
| All literals correspond to immutable data types, and hence the object's identity |
| is less important than its value. Multiple evaluations of literals with the |
| same value (either the same occurrence in the program text or a different |
| occurrence) may obtain the same object or a different object with the same |
| value. |
| |
| |
| .. _parenthesized: |
| |
| Parenthesized forms |
| ------------------- |
| |
| .. index:: single: parenthesized form |
| |
| A parenthesized form is an optional expression list enclosed in parentheses: |
| |
| .. productionlist:: |
| parenth_form: "(" [`expression_list`] ")" |
| |
| A parenthesized expression list yields whatever that expression list yields: if |
| the list contains at least one comma, it yields a tuple; otherwise, it yields |
| the single expression that makes up the expression list. |
| |
| .. index:: pair: empty; tuple |
| |
| An empty pair of parentheses yields an empty tuple object. Since tuples are |
| immutable, the rules for literals apply (i.e., two occurrences of the empty |
| tuple may or may not yield the same object). |
| |
| .. index:: |
| single: comma |
| pair: tuple; display |
| |
| Note that tuples are not formed by the parentheses, but rather by use of the |
| comma operator. The exception is the empty tuple, for which parentheses *are* |
| required --- allowing unparenthesized "nothing" in expressions would cause |
| ambiguities and allow common typos to pass uncaught. |
| |
| |
| .. _comprehensions: |
| |
| Displays for lists, sets and dictionaries |
| ----------------------------------------- |
| |
| For constructing a list, a set or a dictionary Python provides special syntax |
| called "displays", each of them in two flavors: |
| |
| * either the container contents are listed explicitly, or |
| |
| * they are computed via a set of looping and filtering instructions, called a |
| :dfn:`comprehension`. |
| |
| Common syntax elements for comprehensions are: |
| |
| .. productionlist:: |
| comprehension: `expression` `comp_for` |
| comp_for: "for" `target_list` "in" `or_test` [`comp_iter`] |
| comp_iter: `comp_for` | `comp_if` |
| comp_if: "if" `expression_nocond` [`comp_iter`] |
| |
| The comprehension consists of a single expression followed by at least one |
| :keyword:`for` clause and zero or more :keyword:`for` or :keyword:`if` clauses. |
| In this case, the elements of the new container are those that would be produced |
| by considering each of the :keyword:`for` or :keyword:`if` clauses a block, |
| nesting from left to right, and evaluating the expression to produce an element |
| each time the innermost block is reached. |
| |
| Note that the comprehension is executed in a separate scope, so names assigned |
| to in the target list don't "leak" into the enclosing scope. |
| |
| |
| .. _lists: |
| |
| List displays |
| ------------- |
| |
| .. index:: |
| pair: list; display |
| pair: list; comprehensions |
| pair: empty; list |
| object: list |
| |
| A list display is a possibly empty series of expressions enclosed in square |
| brackets: |
| |
| .. productionlist:: |
| list_display: "[" [`expression_list` | `comprehension`] "]" |
| |
| A list display yields a new list object, the contents being specified by either |
| a list of expressions or a comprehension. When a comma-separated list of |
| expressions is supplied, its elements are evaluated from left to right and |
| placed into the list object in that order. When a comprehension is supplied, |
| the list is constructed from the elements resulting from the comprehension. |
| |
| |
| .. _set: |
| |
| Set displays |
| ------------ |
| |
| .. index:: pair: set; display |
| object: set |
| |
| A set display is denoted by curly braces and distinguishable from dictionary |
| displays by the lack of colons separating keys and values: |
| |
| .. productionlist:: |
| set_display: "{" (`expression_list` | `comprehension`) "}" |
| |
| A set display yields a new mutable set object, the contents being specified by |
| either a sequence of expressions or a comprehension. When a comma-separated |
| list of expressions is supplied, its elements are evaluated from left to right |
| and added to the set object. When a comprehension is supplied, the set is |
| constructed from the elements resulting from the comprehension. |
| |
| An empty set cannot be constructed with ``{}``; this literal constructs an empty |
| dictionary. |
| |
| |
| .. _dict: |
| |
| Dictionary displays |
| ------------------- |
| |
| .. index:: pair: dictionary; display |
| key, datum, key/datum pair |
| object: dictionary |
| |
| A dictionary display is a possibly empty series of key/datum pairs enclosed in |
| curly braces: |
| |
| .. productionlist:: |
| dict_display: "{" [`key_datum_list` | `dict_comprehension`] "}" |
| key_datum_list: `key_datum` ("," `key_datum`)* [","] |
| key_datum: `expression` ":" `expression` |
| dict_comprehension: `expression` ":" `expression` `comp_for` |
| |
| A dictionary display yields a new dictionary object. |
| |
| If a comma-separated sequence of key/datum pairs is given, they are evaluated |
| from left to right to define the entries of the dictionary: each key object is |
| used as a key into the dictionary to store the corresponding datum. This means |
| that you can specify the same key multiple times in the key/datum list, and the |
| final dictionary's value for that key will be the last one given. |
| |
| A dict comprehension, in contrast to list and set comprehensions, needs two |
| expressions separated with a colon followed by the usual "for" and "if" clauses. |
| When the comprehension is run, the resulting key and value elements are inserted |
| in the new dictionary in the order they are produced. |
| |
| .. index:: pair: immutable; object |
| hashable |
| |
| Restrictions on the types of the key values are listed earlier in section |
| :ref:`types`. (To summarize, the key type should be :term:`hashable`, which excludes |
| all mutable objects.) Clashes between duplicate keys are not detected; the last |
| datum (textually rightmost in the display) stored for a given key value |
| prevails. |
| |
| |
| .. _genexpr: |
| |
| Generator expressions |
| --------------------- |
| |
| .. index:: pair: generator; expression |
| object: generator |
| |
| A generator expression is a compact generator notation in parentheses: |
| |
| .. productionlist:: |
| generator_expression: "(" `expression` `comp_for` ")" |
| |
| A generator expression yields a new generator object. Its syntax is the same as |
| for comprehensions, except that it is enclosed in parentheses instead of |
| brackets or curly braces. |
| |
| Variables used in the generator expression are evaluated lazily when the |
| :meth:`~generator.__next__` method is called for the generator object (in the same |
| fashion as normal generators). However, the leftmost :keyword:`for` clause is |
| immediately evaluated, so that an error produced by it can be seen before any |
| other possible error in the code that handles the generator expression. |
| Subsequent :keyword:`for` clauses cannot be evaluated immediately since they |
| may depend on the previous :keyword:`for` loop. For example: ``(x*y for x in |
| range(10) for y in bar(x))``. |
| |
| The parentheses can be omitted on calls with only one argument. See section |
| :ref:`calls` for details. |
| |
| |
| .. _yieldexpr: |
| |
| Yield expressions |
| ----------------- |
| |
| .. index:: |
| keyword: yield |
| pair: yield; expression |
| pair: generator; function |
| |
| .. productionlist:: |
| yield_atom: "(" `yield_expression` ")" |
| yield_expression: "yield" [`expression_list` | "from" `expression`] |
| |
| The yield expression is only used when defining a :term:`generator` function and |
| thus can only be used in the body of a function definition. Using a yield |
| expression in a function's body causes that function to be a generator. |
| |
| When a generator function is called, it returns an iterator known as a |
| generator. That generator then controls the execution of the generator function. |
| The execution starts when one of the generator's methods is called. At that |
| time, the execution proceeds to the first yield expression, where it is |
| suspended again, returning the value of :token:`expression_list` to the generator's |
| caller. By suspended, we mean that all local state is retained, including the |
| current bindings of local variables, the instruction pointer, the internal |
| evaluation stack, and the state of any exception handling. When the execution |
| is resumed by calling one of the |
| generator's methods, the function can proceed exactly as if the yield expression |
| were just another external call. The value of the yield expression after |
| resuming depends on the method which resumed the execution. If |
| :meth:`~generator.__next__` is used (typically via either a :keyword:`for` or |
| the :func:`next` builtin) then the result is :const:`None`. Otherwise, if |
| :meth:`~generator.send` is used, then the result will be the value passed in to |
| that method. |
| |
| .. index:: single: coroutine |
| |
| All of this makes generator functions quite similar to coroutines; they yield |
| multiple times, they have more than one entry point and their execution can be |
| suspended. The only difference is that a generator function cannot control |
| where the execution should continue after it yields; the control is always |
| transferred to the generator's caller. |
| |
| Yield expressions are allowed anywhere in a :keyword:`try` construct. If the |
| generator is not resumed before it is |
| finalized (by reaching a zero reference count or by being garbage collected), |
| the generator-iterator's :meth:`~generator.close` method will be called, |
| allowing any pending :keyword:`finally` clauses to execute. |
| |
| When ``yield from <expr>`` is used, it treats the supplied expression as |
| a subiterator. All values produced by that subiterator are passed directly |
| to the caller of the current generator's methods. Any values passed in with |
| :meth:`~generator.send` and any exceptions passed in with |
| :meth:`~generator.throw` are passed to the underlying iterator if it has the |
| appropriate methods. If this is not the case, then :meth:`~generator.send` |
| will raise :exc:`AttributeError` or :exc:`TypeError`, while |
| :meth:`~generator.throw` will just raise the passed in exception immediately. |
| |
| When the underlying iterator is complete, the :attr:`~StopIteration.value` |
| attribute of the raised :exc:`StopIteration` instance becomes the value of |
| the yield expression. It can be either set explicitly when raising |
| :exc:`StopIteration`, or automatically when the sub-iterator is a generator |
| (by returning a value from the sub-generator). |
| |
| .. versionchanged:: 3.3 |
| Added ``yield from <expr>`` to delegate control flow to a subiterator. |
| |
| The parentheses may be omitted when the yield expression is the sole expression |
| on the right hand side of an assignment statement. |
| |
| .. seealso:: |
| |
| :pep:`0255` - Simple Generators |
| The proposal for adding generators and the :keyword:`yield` statement to Python. |
| |
| :pep:`0342` - Coroutines via Enhanced Generators |
| The proposal to enhance the API and syntax of generators, making them |
| usable as simple coroutines. |
| |
| :pep:`0380` - Syntax for Delegating to a Subgenerator |
| The proposal to introduce the :token:`yield_from` syntax, making delegation |
| to sub-generators easy. |
| |
| .. index:: object: generator |
| .. _generator-methods: |
| |
| Generator-iterator methods |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| This subsection describes the methods of a generator iterator. They can |
| be used to control the execution of a generator function. |
| |
| Note that calling any of the generator methods below when the generator |
| is already executing raises a :exc:`ValueError` exception. |
| |
| .. index:: exception: StopIteration |
| |
| |
| .. method:: generator.__next__() |
| |
| Starts the execution of a generator function or resumes it at the last |
| executed yield expression. When a generator function is resumed with a |
| :meth:`~generator.__next__` method, the current yield expression always |
| evaluates to :const:`None`. The execution then continues to the next yield |
| expression, where the generator is suspended again, and the value of the |
| :token:`expression_list` is returned to :meth:`__next__`'s caller. If the |
| generator exits without yielding another value, a :exc:`StopIteration` |
| exception is raised. |
| |
| This method is normally called implicitly, e.g. by a :keyword:`for` loop, or |
| by the built-in :func:`next` function. |
| |
| |
| .. method:: generator.send(value) |
| |
| Resumes the execution and "sends" a value into the generator function. The |
| *value* argument becomes the result of the current yield expression. The |
| :meth:`send` method returns the next value yielded by the generator, or |
| raises :exc:`StopIteration` if the generator exits without yielding another |
| value. When :meth:`send` is called to start the generator, it must be called |
| with :const:`None` as the argument, because there is no yield expression that |
| could receive the value. |
| |
| |
| .. method:: generator.throw(type[, value[, traceback]]) |
| |
| Raises an exception of type ``type`` at the point where the generator was paused, |
| and returns the next value yielded by the generator function. If the generator |
| exits without yielding another value, a :exc:`StopIteration` exception is |
| raised. If the generator function does not catch the passed-in exception, or |
| raises a different exception, then that exception propagates to the caller. |
| |
| .. index:: exception: GeneratorExit |
| |
| |
| .. method:: generator.close() |
| |
| Raises a :exc:`GeneratorExit` at the point where the generator function was |
| paused. If the generator function then exits gracefully, is already closed, |
| or raises :exc:`GeneratorExit` (by not catching the exception), close |
| returns to its caller. If the generator yields a value, a |
| :exc:`RuntimeError` is raised. If the generator raises any other exception, |
| it is propagated to the caller. :meth:`close` does nothing if the generator |
| has already exited due to an exception or normal exit. |
| |
| .. index:: single: yield; examples |
| |
| Examples |
| ^^^^^^^^ |
| |
| Here is a simple example that demonstrates the behavior of generators and |
| generator functions:: |
| |
| >>> def echo(value=None): |
| ... print("Execution starts when 'next()' is called for the first time.") |
| ... try: |
| ... while True: |
| ... try: |
| ... value = (yield value) |
| ... except Exception as e: |
| ... value = e |
| ... finally: |
| ... print("Don't forget to clean up when 'close()' is called.") |
| ... |
| >>> generator = echo(1) |
| >>> print(next(generator)) |
| Execution starts when 'next()' is called for the first time. |
| 1 |
| >>> print(next(generator)) |
| None |
| >>> print(generator.send(2)) |
| 2 |
| >>> generator.throw(TypeError, "spam") |
| TypeError('spam',) |
| >>> generator.close() |
| Don't forget to clean up when 'close()' is called. |
| |
| For examples using ``yield from``, see :ref:`pep-380` in "What's New in |
| Python." |
| |
| |
| .. _primaries: |
| |
| Primaries |
| ========= |
| |
| .. index:: single: primary |
| |
| Primaries represent the most tightly bound operations of the language. Their |
| syntax is: |
| |
| .. productionlist:: |
| primary: `atom` | `attributeref` | `subscription` | `slicing` | `call` |
| |
| |
| .. _attribute-references: |
| |
| Attribute references |
| -------------------- |
| |
| .. index:: pair: attribute; reference |
| |
| An attribute reference is a primary followed by a period and a name: |
| |
| .. productionlist:: |
| attributeref: `primary` "." `identifier` |
| |
| .. index:: |
| exception: AttributeError |
| object: module |
| object: list |
| |
| The primary must evaluate to an object of a type that supports attribute |
| references, which most objects do. This object is then asked to produce the |
| attribute whose name is the identifier. This production can be customized by |
| overriding the :meth:`__getattr__` method. If this attribute is not available, |
| the exception :exc:`AttributeError` is raised. Otherwise, the type and value of |
| the object produced is determined by the object. Multiple evaluations of the |
| same attribute reference may yield different objects. |
| |
| |
| .. _subscriptions: |
| |
| Subscriptions |
| ------------- |
| |
| .. index:: single: subscription |
| |
| .. index:: |
| object: sequence |
| object: mapping |
| object: string |
| object: tuple |
| object: list |
| object: dictionary |
| pair: sequence; item |
| |
| A subscription selects an item of a sequence (string, tuple or list) or mapping |
| (dictionary) object: |
| |
| .. productionlist:: |
| subscription: `primary` "[" `expression_list` "]" |
| |
| The primary must evaluate to an object that supports subscription (lists or |
| dictionaries for example). User-defined objects can support subscription by |
| defining a :meth:`__getitem__` method. |
| |
| For built-in objects, there are two types of objects that support subscription: |
| |
| If the primary is a mapping, the expression list must evaluate to an object |
| whose value is one of the keys of the mapping, and the subscription selects the |
| value in the mapping that corresponds to that key. (The expression list is a |
| tuple except if it has exactly one item.) |
| |
| If the primary is a sequence, the expression (list) must evaluate to an integer |
| or a slice (as discussed in the following section). |
| |
| The formal syntax makes no special provision for negative indices in |
| sequences; however, built-in sequences all provide a :meth:`__getitem__` |
| method that interprets negative indices by adding the length of the sequence |
| to the index (so that ``x[-1]`` selects the last item of ``x``). The |
| resulting value must be a nonnegative integer less than the number of items in |
| the sequence, and the subscription selects the item whose index is that value |
| (counting from zero). Since the support for negative indices and slicing |
| occurs in the object's :meth:`__getitem__` method, subclasses overriding |
| this method will need to explicitly add that support. |
| |
| .. index:: |
| single: character |
| pair: string; item |
| |
| A string's items are characters. A character is not a separate data type but a |
| string of exactly one character. |
| |
| |
| .. _slicings: |
| |
| Slicings |
| -------- |
| |
| .. index:: |
| single: slicing |
| single: slice |
| |
| .. index:: |
| object: sequence |
| object: string |
| object: tuple |
| object: list |
| |
| A slicing selects a range of items in a sequence object (e.g., a string, tuple |
| or list). Slicings may be used as expressions or as targets in assignment or |
| :keyword:`del` statements. The syntax for a slicing: |
| |
| .. productionlist:: |
| slicing: `primary` "[" `slice_list` "]" |
| slice_list: `slice_item` ("," `slice_item`)* [","] |
| slice_item: `expression` | `proper_slice` |
| proper_slice: [`lower_bound`] ":" [`upper_bound`] [ ":" [`stride`] ] |
| lower_bound: `expression` |
| upper_bound: `expression` |
| stride: `expression` |
| |
| There is ambiguity in the formal syntax here: anything that looks like an |
| expression list also looks like a slice list, so any subscription can be |
| interpreted as a slicing. Rather than further complicating the syntax, this is |
| disambiguated by defining that in this case the interpretation as a subscription |
| takes priority over the interpretation as a slicing (this is the case if the |
| slice list contains no proper slice). |
| |
| .. index:: |
| single: start (slice object attribute) |
| single: stop (slice object attribute) |
| single: step (slice object attribute) |
| |
| The semantics for a slicing are as follows. The primary is indexed (using the |
| same :meth:`__getitem__` method as |
| normal subscription) with a key that is constructed from the slice list, as |
| follows. If the slice list contains at least one comma, the key is a tuple |
| containing the conversion of the slice items; otherwise, the conversion of the |
| lone slice item is the key. The conversion of a slice item that is an |
| expression is that expression. The conversion of a proper slice is a slice |
| object (see section :ref:`types`) whose :attr:`~slice.start`, |
| :attr:`~slice.stop` and :attr:`~slice.step` attributes are the values of the |
| expressions given as lower bound, upper bound and stride, respectively, |
| substituting ``None`` for missing expressions. |
| |
| |
| .. index:: |
| object: callable |
| single: call |
| single: argument; call semantics |
| |
| .. _calls: |
| |
| Calls |
| ----- |
| |
| A call calls a callable object (e.g., a :term:`function`) with a possibly empty |
| series of :term:`arguments <argument>`: |
| |
| .. productionlist:: |
| call: `primary` "(" [`argument_list` [","] | `comprehension`] ")" |
| argument_list: `positional_arguments` ["," `keyword_arguments`] |
| : ["," "*" `expression`] ["," `keyword_arguments`] |
| : ["," "**" `expression`] |
| : | `keyword_arguments` ["," "*" `expression`] |
| : ["," `keyword_arguments`] ["," "**" `expression`] |
| : | "*" `expression` ["," `keyword_arguments`] ["," "**" `expression`] |
| : | "**" `expression` |
| positional_arguments: `expression` ("," `expression`)* |
| keyword_arguments: `keyword_item` ("," `keyword_item`)* |
| keyword_item: `identifier` "=" `expression` |
| |
| An optional trailing comma may be present after the positional and keyword arguments |
| but does not affect the semantics. |
| |
| .. index:: |
| single: parameter; call semantics |
| |
| The primary must evaluate to a callable object (user-defined functions, built-in |
| functions, methods of built-in objects, class objects, methods of class |
| instances, and all objects having a :meth:`__call__` method are callable). All |
| argument expressions are evaluated before the call is attempted. Please refer |
| to section :ref:`function` for the syntax of formal :term:`parameter` lists. |
| |
| .. XXX update with kwonly args PEP |
| |
| If keyword arguments are present, they are first converted to positional |
| arguments, as follows. First, a list of unfilled slots is created for the |
| formal parameters. If there are N positional arguments, they are placed in the |
| first N slots. Next, for each keyword argument, the identifier is used to |
| determine the corresponding slot (if the identifier is the same as the first |
| formal parameter name, the first slot is used, and so on). If the slot is |
| already filled, a :exc:`TypeError` exception is raised. Otherwise, the value of |
| the argument is placed in the slot, filling it (even if the expression is |
| ``None``, it fills the slot). When all arguments have been processed, the slots |
| that are still unfilled are filled with the corresponding default value from the |
| function definition. (Default values are calculated, once, when the function is |
| defined; thus, a mutable object such as a list or dictionary used as default |
| value will be shared by all calls that don't specify an argument value for the |
| corresponding slot; this should usually be avoided.) If there are any unfilled |
| slots for which no default value is specified, a :exc:`TypeError` exception is |
| raised. Otherwise, the list of filled slots is used as the argument list for |
| the call. |
| |
| .. impl-detail:: |
| |
| An implementation may provide built-in functions whose positional parameters |
| do not have names, even if they are 'named' for the purpose of documentation, |
| and which therefore cannot be supplied by keyword. In CPython, this is the |
| case for functions implemented in C that use :c:func:`PyArg_ParseTuple` to |
| parse their arguments. |
| |
| If there are more positional arguments than there are formal parameter slots, a |
| :exc:`TypeError` exception is raised, unless a formal parameter using the syntax |
| ``*identifier`` is present; in this case, that formal parameter receives a tuple |
| containing the excess positional arguments (or an empty tuple if there were no |
| excess positional arguments). |
| |
| If any keyword argument does not correspond to a formal parameter name, a |
| :exc:`TypeError` exception is raised, unless a formal parameter using the syntax |
| ``**identifier`` is present; in this case, that formal parameter receives a |
| dictionary containing the excess keyword arguments (using the keywords as keys |
| and the argument values as corresponding values), or a (new) empty dictionary if |
| there were no excess keyword arguments. |
| |
| .. index:: |
| single: *; in function calls |
| |
| If the syntax ``*expression`` appears in the function call, ``expression`` must |
| evaluate to an iterable. Elements from this iterable are treated as if they |
| were additional positional arguments; if there are positional arguments |
| *x1*, ..., *xN*, and ``expression`` evaluates to a sequence *y1*, ..., *yM*, |
| this is equivalent to a call with M+N positional arguments *x1*, ..., *xN*, |
| *y1*, ..., *yM*. |
| |
| A consequence of this is that although the ``*expression`` syntax may appear |
| *after* some keyword arguments, it is processed *before* the keyword arguments |
| (and the ``**expression`` argument, if any -- see below). So:: |
| |
| >>> def f(a, b): |
| ... print(a, b) |
| ... |
| >>> f(b=1, *(2,)) |
| 2 1 |
| >>> f(a=1, *(2,)) |
| Traceback (most recent call last): |
| File "<stdin>", line 1, in ? |
| TypeError: f() got multiple values for keyword argument 'a' |
| >>> f(1, *(2,)) |
| 1 2 |
| |
| It is unusual for both keyword arguments and the ``*expression`` syntax to be |
| used in the same call, so in practice this confusion does not arise. |
| |
| .. index:: |
| single: **; in function calls |
| |
| If the syntax ``**expression`` appears in the function call, ``expression`` must |
| evaluate to a mapping, the contents of which are treated as additional keyword |
| arguments. In the case of a keyword appearing in both ``expression`` and as an |
| explicit keyword argument, a :exc:`TypeError` exception is raised. |
| |
| Formal parameters using the syntax ``*identifier`` or ``**identifier`` cannot be |
| used as positional argument slots or as keyword argument names. |
| |
| A call always returns some value, possibly ``None``, unless it raises an |
| exception. How this value is computed depends on the type of the callable |
| object. |
| |
| If it is--- |
| |
| a user-defined function: |
| .. index:: |
| pair: function; call |
| triple: user-defined; function; call |
| object: user-defined function |
| object: function |
| |
| The code block for the function is executed, passing it the argument list. The |
| first thing the code block will do is bind the formal parameters to the |
| arguments; this is described in section :ref:`function`. When the code block |
| executes a :keyword:`return` statement, this specifies the return value of the |
| function call. |
| |
| a built-in function or method: |
| .. index:: |
| pair: function; call |
| pair: built-in function; call |
| pair: method; call |
| pair: built-in method; call |
| object: built-in method |
| object: built-in function |
| object: method |
| object: function |
| |
| The result is up to the interpreter; see :ref:`built-in-funcs` for the |
| descriptions of built-in functions and methods. |
| |
| a class object: |
| .. index:: |
| object: class |
| pair: class object; call |
| |
| A new instance of that class is returned. |
| |
| a class instance method: |
| .. index:: |
| object: class instance |
| object: instance |
| pair: class instance; call |
| |
| The corresponding user-defined function is called, with an argument list that is |
| one longer than the argument list of the call: the instance becomes the first |
| argument. |
| |
| a class instance: |
| .. index:: |
| pair: instance; call |
| single: __call__() (object method) |
| |
| The class must define a :meth:`__call__` method; the effect is then the same as |
| if that method was called. |
| |
| |
| .. _await: |
| |
| Await expression |
| ================ |
| |
| Suspend the execution of :term:`coroutine` on an :term:`awaitable` object. |
| Can only be used inside a :term:`coroutine function`. |
| |
| .. productionlist:: |
| await: ["await"] `primary` |
| |
| .. versionadded:: 3.5 |
| |
| |
| .. _power: |
| |
| The power operator |
| ================== |
| |
| The power operator binds more tightly than unary operators on its left; it binds |
| less tightly than unary operators on its right. The syntax is: |
| |
| .. productionlist:: |
| power: `await` ["**" `u_expr`] |
| |
| Thus, in an unparenthesized sequence of power and unary operators, the operators |
| are evaluated from right to left (this does not constrain the evaluation order |
| for the operands): ``-1**2`` results in ``-1``. |
| |
| The power operator has the same semantics as the built-in :func:`pow` function, |
| when called with two arguments: it yields its left argument raised to the power |
| of its right argument. The numeric arguments are first converted to a common |
| type, and the result is of that type. |
| |
| For int operands, the result has the same type as the operands unless the second |
| argument is negative; in that case, all arguments are converted to float and a |
| float result is delivered. For example, ``10**2`` returns ``100``, but |
| ``10**-2`` returns ``0.01``. |
| |
| Raising ``0.0`` to a negative power results in a :exc:`ZeroDivisionError`. |
| Raising a negative number to a fractional power results in a :class:`complex` |
| number. (In earlier versions it raised a :exc:`ValueError`.) |
| |
| |
| .. _unary: |
| |
| Unary arithmetic and bitwise operations |
| ======================================= |
| |
| .. index:: |
| triple: unary; arithmetic; operation |
| triple: unary; bitwise; operation |
| |
| All unary arithmetic and bitwise operations have the same priority: |
| |
| .. productionlist:: |
| u_expr: `power` | "-" `u_expr` | "+" `u_expr` | "~" `u_expr` |
| |
| .. index:: |
| single: negation |
| single: minus |
| |
| The unary ``-`` (minus) operator yields the negation of its numeric argument. |
| |
| .. index:: single: plus |
| |
| The unary ``+`` (plus) operator yields its numeric argument unchanged. |
| |
| .. index:: single: inversion |
| |
| |
| The unary ``~`` (invert) operator yields the bitwise inversion of its integer |
| argument. The bitwise inversion of ``x`` is defined as ``-(x+1)``. It only |
| applies to integral numbers. |
| |
| .. index:: exception: TypeError |
| |
| In all three cases, if the argument does not have the proper type, a |
| :exc:`TypeError` exception is raised. |
| |
| |
| .. _binary: |
| |
| Binary arithmetic operations |
| ============================ |
| |
| .. index:: triple: binary; arithmetic; operation |
| |
| The binary arithmetic operations have the conventional priority levels. Note |
| that some of these operations also apply to certain non-numeric types. Apart |
| from the power operator, there are only two levels, one for multiplicative |
| operators and one for additive operators: |
| |
| .. productionlist:: |
| m_expr: `u_expr` | `m_expr` "*" `u_expr` | `m_expr` "@" `m_expr` | |
| : `m_expr` "//" `u_expr`| `m_expr` "/" `u_expr` | |
| : `m_expr` "%" `u_expr` |
| a_expr: `m_expr` | `a_expr` "+" `m_expr` | `a_expr` "-" `m_expr` |
| |
| .. index:: single: multiplication |
| |
| The ``*`` (multiplication) operator yields the product of its arguments. The |
| arguments must either both be numbers, or one argument must be an integer and |
| the other must be a sequence. In the former case, the numbers are converted to a |
| common type and then multiplied together. In the latter case, sequence |
| repetition is performed; a negative repetition factor yields an empty sequence. |
| |
| .. index:: single: matrix multiplication |
| |
| The ``@`` (at) operator is intended to be used for matrix multiplication. No |
| builtin Python types implement this operator. |
| |
| .. versionadded:: 3.5 |
| |
| .. index:: |
| exception: ZeroDivisionError |
| single: division |
| |
| The ``/`` (division) and ``//`` (floor division) operators yield the quotient of |
| their arguments. The numeric arguments are first converted to a common type. |
| Division of integers yields a float, while floor division of integers results in an |
| integer; the result is that of mathematical division with the 'floor' function |
| applied to the result. Division by zero raises the :exc:`ZeroDivisionError` |
| exception. |
| |
| .. index:: single: modulo |
| |
| The ``%`` (modulo) operator yields the remainder from the division of the first |
| argument by the second. The numeric arguments are first converted to a common |
| type. A zero right argument raises the :exc:`ZeroDivisionError` exception. The |
| arguments may be floating point numbers, e.g., ``3.14%0.7`` equals ``0.34`` |
| (since ``3.14`` equals ``4*0.7 + 0.34``.) The modulo operator always yields a |
| result with the same sign as its second operand (or zero); the absolute value of |
| the result is strictly smaller than the absolute value of the second operand |
| [#]_. |
| |
| The floor division and modulo operators are connected by the following |
| identity: ``x == (x//y)*y + (x%y)``. Floor division and modulo are also |
| connected with the built-in function :func:`divmod`: ``divmod(x, y) == (x//y, |
| x%y)``. [#]_. |
| |
| In addition to performing the modulo operation on numbers, the ``%`` operator is |
| also overloaded by string objects to perform old-style string formatting (also |
| known as interpolation). The syntax for string formatting is described in the |
| Python Library Reference, section :ref:`old-string-formatting`. |
| |
| The floor division operator, the modulo operator, and the :func:`divmod` |
| function are not defined for complex numbers. Instead, convert to a floating |
| point number using the :func:`abs` function if appropriate. |
| |
| .. index:: single: addition |
| |
| The ``+`` (addition) operator yields the sum of its arguments. The arguments |
| must either both be numbers or both be sequences of the same type. In the |
| former case, the numbers are converted to a common type and then added together. |
| In the latter case, the sequences are concatenated. |
| |
| .. index:: single: subtraction |
| |
| The ``-`` (subtraction) operator yields the difference of its arguments. The |
| numeric arguments are first converted to a common type. |
| |
| |
| .. _shifting: |
| |
| Shifting operations |
| =================== |
| |
| .. index:: pair: shifting; operation |
| |
| The shifting operations have lower priority than the arithmetic operations: |
| |
| .. productionlist:: |
| shift_expr: `a_expr` | `shift_expr` ( "<<" | ">>" ) `a_expr` |
| |
| These operators accept integers as arguments. They shift the first argument to |
| the left or right by the number of bits given by the second argument. |
| |
| .. index:: exception: ValueError |
| |
| A right shift by *n* bits is defined as floor division by ``pow(2,n)``. A left |
| shift by *n* bits is defined as multiplication with ``pow(2,n)``. |
| |
| .. note:: |
| |
| In the current implementation, the right-hand operand is required |
| to be at most :attr:`sys.maxsize`. If the right-hand operand is larger than |
| :attr:`sys.maxsize` an :exc:`OverflowError` exception is raised. |
| |
| .. _bitwise: |
| |
| Binary bitwise operations |
| ========================= |
| |
| .. index:: triple: binary; bitwise; operation |
| |
| Each of the three bitwise operations has a different priority level: |
| |
| .. productionlist:: |
| and_expr: `shift_expr` | `and_expr` "&" `shift_expr` |
| xor_expr: `and_expr` | `xor_expr` "^" `and_expr` |
| or_expr: `xor_expr` | `or_expr` "|" `xor_expr` |
| |
| .. index:: pair: bitwise; and |
| |
| The ``&`` operator yields the bitwise AND of its arguments, which must be |
| integers. |
| |
| .. index:: |
| pair: bitwise; xor |
| pair: exclusive; or |
| |
| The ``^`` operator yields the bitwise XOR (exclusive OR) of its arguments, which |
| must be integers. |
| |
| .. index:: |
| pair: bitwise; or |
| pair: inclusive; or |
| |
| The ``|`` operator yields the bitwise (inclusive) OR of its arguments, which |
| must be integers. |
| |
| |
| .. _comparisons: |
| |
| Comparisons |
| =========== |
| |
| .. index:: single: comparison |
| |
| .. index:: pair: C; language |
| |
| Unlike C, all comparison operations in Python have the same priority, which is |
| lower than that of any arithmetic, shifting or bitwise operation. Also unlike |
| C, expressions like ``a < b < c`` have the interpretation that is conventional |
| in mathematics: |
| |
| .. productionlist:: |
| comparison: `or_expr` ( `comp_operator` `or_expr` )* |
| comp_operator: "<" | ">" | "==" | ">=" | "<=" | "!=" |
| : | "is" ["not"] | ["not"] "in" |
| |
| Comparisons yield boolean values: ``True`` or ``False``. |
| |
| .. index:: pair: chaining; comparisons |
| |
| Comparisons can be chained arbitrarily, e.g., ``x < y <= z`` is equivalent to |
| ``x < y and y <= z``, except that ``y`` is evaluated only once (but in both |
| cases ``z`` is not evaluated at all when ``x < y`` is found to be false). |
| |
| Formally, if *a*, *b*, *c*, ..., *y*, *z* are expressions and *op1*, *op2*, ..., |
| *opN* are comparison operators, then ``a op1 b op2 c ... y opN z`` is equivalent |
| to ``a op1 b and b op2 c and ... y opN z``, except that each expression is |
| evaluated at most once. |
| |
| Note that ``a op1 b op2 c`` doesn't imply any kind of comparison between *a* and |
| *c*, so that, e.g., ``x < y > z`` is perfectly legal (though perhaps not |
| pretty). |
| |
| Value comparisons |
| ----------------- |
| |
| The operators ``<``, ``>``, ``==``, ``>=``, ``<=``, and ``!=`` compare the |
| values of two objects. The objects do not need to have the same type. |
| |
| Chapter :ref:`objects` states that objects have a value (in addition to type |
| and identity). The value of an object is a rather abstract notion in Python: |
| For example, there is no canonical access method for an object's value. Also, |
| there is no requirement that the value of an object should be constructed in a |
| particular way, e.g. comprised of all its data attributes. Comparison operators |
| implement a particular notion of what the value of an object is. One can think |
| of them as defining the value of an object indirectly, by means of their |
| comparison implementation. |
| |
| Because all types are (direct or indirect) subtypes of :class:`object`, they |
| inherit the default comparison behavior from :class:`object`. Types can |
| customize their comparison behavior by implementing |
| :dfn:`rich comparison methods` like :meth:`__lt__`, described in |
| :ref:`customization`. |
| |
| The default behavior for equality comparison (``==`` and ``!=``) is based on |
| the identity of the objects. Hence, equality comparison of instances with the |
| same identity results in equality, and equality comparison of instances with |
| different identities results in inequality. A motivation for this default |
| behavior is the desire that all objects should be reflexive (i.e. ``x is y`` |
| implies ``x == y``). |
| |
| A default order comparison (``<``, ``>``, ``<=``, and ``>=``) is not provided; |
| an attempt raises :exc:`TypeError`. A motivation for this default behavior is |
| the lack of a similar invariant as for equality. |
| |
| The behavior of the default equality comparison, that instances with different |
| identities are always unequal, may be in contrast to what types will need that |
| have a sensible definition of object value and value-based equality. Such |
| types will need to customize their comparison behavior, and in fact, a number |
| of built-in types have done that. |
| |
| The following list describes the comparison behavior of the most important |
| built-in types. |
| |
| * Numbers of built-in numeric types (:ref:`typesnumeric`) and of the standard |
| library types :class:`fractions.Fraction` and :class:`decimal.Decimal` can be |
| compared within and across their types, with the restriction that complex |
| numbers do not support order comparison. Within the limits of the types |
| involved, they compare mathematically (algorithmically) correct without loss |
| of precision. |
| |
| The not-a-number values :const:`float('NaN')` and :const:`Decimal('NaN')` |
| are special. They are identical to themselves (``x is x`` is true) but |
| are not equal to themselves (``x == x`` is false). Additionally, |
| comparing any number to a not-a-number value |
| will return ``False``. For example, both ``3 < float('NaN')`` and |
| ``float('NaN') < 3`` will return ``False``. |
| |
| * Binary sequences (instances of :class:`bytes` or :class:`bytearray`) can be |
| compared within and across their types. They compare lexicographically using |
| the numeric values of their elements. |
| |
| * Strings (instances of :class:`str`) compare lexicographically using the |
| numerical Unicode code points (the result of the built-in function |
| :func:`ord`) of their characters. [#]_ |
| |
| Strings and binary sequences cannot be directly compared. |
| |
| * Sequences (instances of :class:`tuple`, :class:`list`, or :class:`range`) can |
| be compared only within each of their types, with the restriction that ranges |
| do not support order comparison. Equality comparison across these types |
| results in unequality, and ordering comparison across these types raises |
| :exc:`TypeError`. |
| |
| Sequences compare lexicographically using comparison of corresponding |
| elements, whereby reflexivity of the elements is enforced. |
| |
| In enforcing reflexivity of elements, the comparison of collections assumes |
| that for a collection element ``x``, ``x == x`` is always true. Based on |
| that assumption, element identity is compared first, and element comparison |
| is performed only for distinct elements. This approach yields the same |
| result as a strict element comparison would, if the compared elements are |
| reflexive. For non-reflexive elements, the result is different than for |
| strict element comparison, and may be surprising: The non-reflexive |
| not-a-number values for example result in the following comparison behavior |
| when used in a list:: |
| |
| >>> nan = float('NaN') |
| >>> nan is nan |
| True |
| >>> nan == nan |
| False <-- the defined non-reflexive behavior of NaN |
| >>> [nan] == [nan] |
| True <-- list enforces reflexivity and tests identity first |
| |
| Lexicographical comparison between built-in collections works as follows: |
| |
| - For two collections to compare equal, they must be of the same type, have |
| the same length, and each pair of corresponding elements must compare |
| equal (for example, ``[1,2] == (1,2)`` is false because the type is not the |
| same). |
| |
| - Collections that support order comparison are ordered the same as their |
| first unequal elements (for example, ``[1,2,x] <= [1,2,y]`` has the same |
| value as ``x <= y``). If a corresponding element does not exist, the |
| shorter collection is ordered first (for example, ``[1,2] < [1,2,3]`` is |
| true). |
| |
| * Mappings (instances of :class:`dict`) compare equal if and only if they have |
| equal `(key, value)` pairs. Equality comparison of the keys and elements |
| enforces reflexivity. |
| |
| Order comparisons (``<``, ``>``, ``<=``, and ``>=``) raise :exc:`TypeError`. |
| |
| * Sets (instances of :class:`set` or :class:`frozenset`) can be compared within |
| and across their types. |
| |
| They define order |
| comparison operators to mean subset and superset tests. Those relations do |
| not define total orderings (for example, the two sets ``{1,2}`` and ``{2,3}`` |
| are not equal, nor subsets of one another, nor supersets of one |
| another). Accordingly, sets are not appropriate arguments for functions |
| which depend on total ordering (for example, :func:`min`, :func:`max`, and |
| :func:`sorted` produce undefined results given a list of sets as inputs). |
| |
| Comparison of sets enforces reflexivity of its elements. |
| |
| * Most other built-in types have no comparison methods implemented, so they |
| inherit the default comparison behavior. |
| |
| User-defined classes that customize their comparison behavior should follow |
| some consistency rules, if possible: |
| |
| * Equality comparison should be reflexive. |
| In other words, identical objects should compare equal: |
| |
| ``x is y`` implies ``x == y`` |
| |
| * Comparison should be symmetric. |
| In other words, the following expressions should have the same result: |
| |
| ``x == y`` and ``y == x`` |
| |
| ``x != y`` and ``y != x`` |
| |
| ``x < y`` and ``y > x`` |
| |
| ``x <= y`` and ``y >= x`` |
| |
| * Comparison should be transitive. |
| The following (non-exhaustive) examples illustrate that: |
| |
| ``x > y and y > z`` implies ``x > z`` |
| |
| ``x < y and y <= z`` implies ``x < z`` |
| |
| * Inverse comparison should result in the boolean negation. |
| In other words, the following expressions should have the same result: |
| |
| ``x == y`` and ``not x != y`` |
| |
| ``x < y`` and ``not x >= y`` (for total ordering) |
| |
| ``x > y`` and ``not x <= y`` (for total ordering) |
| |
| The last two expressions apply to totally ordered collections (e.g. to |
| sequences, but not to sets or mappings). See also the |
| :func:`~functools.total_ordering` decorator. |
| |
| Python does not enforce these consistency rules. In fact, the not-a-number |
| values are an example for not following these rules. |
| |
| |
| .. _in: |
| .. _not in: |
| .. _membership-test-details: |
| |
| Membership test operations |
| -------------------------- |
| |
| The operators :keyword:`in` and :keyword:`not in` test for membership. ``x in |
| s`` evaluates to true if *x* is a member of *s*, and false otherwise. ``x not |
| in s`` returns the negation of ``x in s``. All built-in sequences and set types |
| support this as well as dictionary, for which :keyword:`in` tests whether the |
| dictionary has a given key. For container types such as list, tuple, set, |
| frozenset, dict, or collections.deque, the expression ``x in y`` is equivalent |
| to ``any(x is e or x == e for e in y)``. |
| |
| For the string and bytes types, ``x in y`` is true if and only if *x* is a |
| substring of *y*. An equivalent test is ``y.find(x) != -1``. Empty strings are |
| always considered to be a substring of any other string, so ``"" in "abc"`` will |
| return ``True``. |
| |
| For user-defined classes which define the :meth:`__contains__` method, ``x in |
| y`` is true if and only if ``y.__contains__(x)`` is true. |
| |
| For user-defined classes which do not define :meth:`__contains__` but do define |
| :meth:`__iter__`, ``x in y`` is true if some value ``z`` with ``x == z`` is |
| produced while iterating over ``y``. If an exception is raised during the |
| iteration, it is as if :keyword:`in` raised that exception. |
| |
| Lastly, the old-style iteration protocol is tried: if a class defines |
| :meth:`__getitem__`, ``x in y`` is true if and only if there is a non-negative |
| integer index *i* such that ``x == y[i]``, and all lower integer indices do not |
| raise :exc:`IndexError` exception. (If any other exception is raised, it is as |
| if :keyword:`in` raised that exception). |
| |
| .. index:: |
| operator: in |
| operator: not in |
| pair: membership; test |
| object: sequence |
| |
| The operator :keyword:`not in` is defined to have the inverse true value of |
| :keyword:`in`. |
| |
| .. index:: |
| operator: is |
| operator: is not |
| pair: identity; test |
| |
| |
| .. _is: |
| .. _is not: |
| |
| Identity comparisons |
| -------------------- |
| |
| The operators :keyword:`is` and :keyword:`is not` test for object identity: ``x |
| is y`` is true if and only if *x* and *y* are the same object. ``x is not y`` |
| yields the inverse truth value. [#]_ |
| |
| |
| .. _booleans: |
| .. _and: |
| .. _or: |
| .. _not: |
| |
| Boolean operations |
| ================== |
| |
| .. index:: |
| pair: Conditional; expression |
| pair: Boolean; operation |
| |
| .. productionlist:: |
| or_test: `and_test` | `or_test` "or" `and_test` |
| and_test: `not_test` | `and_test` "and" `not_test` |
| not_test: `comparison` | "not" `not_test` |
| |
| In the context of Boolean operations, and also when expressions are used by |
| control flow statements, the following values are interpreted as false: |
| ``False``, ``None``, numeric zero of all types, and empty strings and containers |
| (including strings, tuples, lists, dictionaries, sets and frozensets). All |
| other values are interpreted as true. User-defined objects can customize their |
| truth value by providing a :meth:`__bool__` method. |
| |
| .. index:: operator: not |
| |
| The operator :keyword:`not` yields ``True`` if its argument is false, ``False`` |
| otherwise. |
| |
| .. index:: operator: and |
| |
| The expression ``x and y`` first evaluates *x*; if *x* is false, its value is |
| returned; otherwise, *y* is evaluated and the resulting value is returned. |
| |
| .. index:: operator: or |
| |
| The expression ``x or y`` first evaluates *x*; if *x* is true, its value is |
| returned; otherwise, *y* is evaluated and the resulting value is returned. |
| |
| (Note that neither :keyword:`and` nor :keyword:`or` restrict the value and type |
| they return to ``False`` and ``True``, but rather return the last evaluated |
| argument. This is sometimes useful, e.g., if ``s`` is a string that should be |
| replaced by a default value if it is empty, the expression ``s or 'foo'`` yields |
| the desired value. Because :keyword:`not` has to create a new value, it |
| returns a boolean value regardless of the type of its argument |
| (for example, ``not 'foo'`` produces ``False`` rather than ``''``.) |
| |
| |
| Conditional expressions |
| ======================= |
| |
| .. index:: |
| pair: conditional; expression |
| pair: ternary; operator |
| |
| .. productionlist:: |
| conditional_expression: `or_test` ["if" `or_test` "else" `expression`] |
| expression: `conditional_expression` | `lambda_expr` |
| expression_nocond: `or_test` | `lambda_expr_nocond` |
| |
| Conditional expressions (sometimes called a "ternary operator") have the lowest |
| priority of all Python operations. |
| |
| The expression ``x if C else y`` first evaluates the condition, *C* rather than *x*. |
| If *C* is true, *x* is evaluated and its value is returned; otherwise, *y* is |
| evaluated and its value is returned. |
| |
| See :pep:`308` for more details about conditional expressions. |
| |
| |
| .. _lambdas: |
| .. _lambda: |
| |
| Lambdas |
| ======= |
| |
| .. index:: |
| pair: lambda; expression |
| pair: lambda; form |
| pair: anonymous; function |
| |
| .. productionlist:: |
| lambda_expr: "lambda" [`parameter_list`]: `expression` |
| lambda_expr_nocond: "lambda" [`parameter_list`]: `expression_nocond` |
| |
| Lambda expressions (sometimes called lambda forms) are used to create anonymous |
| functions. The expression ``lambda arguments: expression`` yields a function |
| object. The unnamed object behaves like a function object defined with :: |
| |
| def <lambda>(arguments): |
| return expression |
| |
| See section :ref:`function` for the syntax of parameter lists. Note that |
| functions created with lambda expressions cannot contain statements or |
| annotations. |
| |
| |
| .. _exprlists: |
| |
| Expression lists |
| ================ |
| |
| .. index:: pair: expression; list |
| |
| .. productionlist:: |
| expression_list: `expression` ( "," `expression` )* [","] |
| |
| .. index:: object: tuple |
| |
| An expression list containing at least one comma yields a tuple. The length of |
| the tuple is the number of expressions in the list. The expressions are |
| evaluated from left to right. |
| |
| .. index:: pair: trailing; comma |
| |
| The trailing comma is required only to create a single tuple (a.k.a. a |
| *singleton*); it is optional in all other cases. A single expression without a |
| trailing comma doesn't create a tuple, but rather yields the value of that |
| expression. (To create an empty tuple, use an empty pair of parentheses: |
| ``()``.) |
| |
| |
| .. _evalorder: |
| |
| Evaluation order |
| ================ |
| |
| .. index:: pair: evaluation; order |
| |
| Python evaluates expressions from left to right. Notice that while evaluating |
| an assignment, the right-hand side is evaluated before the left-hand side. |
| |
| In the following lines, expressions will be evaluated in the arithmetic order of |
| their suffixes:: |
| |
| expr1, expr2, expr3, expr4 |
| (expr1, expr2, expr3, expr4) |
| {expr1: expr2, expr3: expr4} |
| expr1 + expr2 * (expr3 - expr4) |
| expr1(expr2, expr3, *expr4, **expr5) |
| expr3, expr4 = expr1, expr2 |
| |
| |
| .. _operator-summary: |
| |
| Operator precedence |
| =================== |
| |
| .. index:: pair: operator; precedence |
| |
| The following table summarizes the operator precedence in Python, from lowest |
| precedence (least binding) to highest precedence (most binding). Operators in |
| the same box have the same precedence. Unless the syntax is explicitly given, |
| operators are binary. Operators in the same box group left to right (except for |
| exponentiation, which groups from right to left). |
| |
| Note that comparisons, membership tests, and identity tests, all have the same |
| precedence and have a left-to-right chaining feature as described in the |
| :ref:`comparisons` section. |
| |
| |
| +-----------------------------------------------+-------------------------------------+ |
| | Operator | Description | |
| +===============================================+=====================================+ |
| | :keyword:`lambda` | Lambda expression | |
| +-----------------------------------------------+-------------------------------------+ |
| | :keyword:`if` -- :keyword:`else` | Conditional expression | |
| +-----------------------------------------------+-------------------------------------+ |
| | :keyword:`or` | Boolean OR | |
| +-----------------------------------------------+-------------------------------------+ |
| | :keyword:`and` | Boolean AND | |
| +-----------------------------------------------+-------------------------------------+ |
| | :keyword:`not` ``x`` | Boolean NOT | |
| +-----------------------------------------------+-------------------------------------+ |
| | :keyword:`in`, :keyword:`not in`, | Comparisons, including membership | |
| | :keyword:`is`, :keyword:`is not`, ``<``, | tests and identity tests | |
| | ``<=``, ``>``, ``>=``, ``!=``, ``==`` | | |
| +-----------------------------------------------+-------------------------------------+ |
| | ``|`` | Bitwise OR | |
| +-----------------------------------------------+-------------------------------------+ |
| | ``^`` | Bitwise XOR | |
| +-----------------------------------------------+-------------------------------------+ |
| | ``&`` | Bitwise AND | |
| +-----------------------------------------------+-------------------------------------+ |
| | ``<<``, ``>>`` | Shifts | |
| +-----------------------------------------------+-------------------------------------+ |
| | ``+``, ``-`` | Addition and subtraction | |
| +-----------------------------------------------+-------------------------------------+ |
| | ``*``, ``@``, ``/``, ``//``, ``%`` | Multiplication, matrix | |
| | | multiplication division, | |
| | | remainder [#]_ | |
| +-----------------------------------------------+-------------------------------------+ |
| | ``+x``, ``-x``, ``~x`` | Positive, negative, bitwise NOT | |
| +-----------------------------------------------+-------------------------------------+ |
| | ``**`` | Exponentiation [#]_ | |
| +-----------------------------------------------+-------------------------------------+ |
| | ``await`` ``x`` | Await expression | |
| +-----------------------------------------------+-------------------------------------+ |
| | ``x[index]``, ``x[index:index]``, | Subscription, slicing, | |
| | ``x(arguments...)``, ``x.attribute`` | call, attribute reference | |
| +-----------------------------------------------+-------------------------------------+ |
| | ``(expressions...)``, | Binding or tuple display, | |
| | ``[expressions...]``, | list display, | |
| | ``{key: value...}``, | dictionary display, | |
| | ``{expressions...}`` | set display | |
| +-----------------------------------------------+-------------------------------------+ |
| |
| |
| .. rubric:: Footnotes |
| |
| .. [#] While ``abs(x%y) < abs(y)`` is true mathematically, for floats it may not be |
| true numerically due to roundoff. For example, and assuming a platform on which |
| a Python float is an IEEE 754 double-precision number, in order that ``-1e-100 % |
| 1e100`` have the same sign as ``1e100``, the computed result is ``-1e-100 + |
| 1e100``, which is numerically exactly equal to ``1e100``. The function |
| :func:`math.fmod` returns a result whose sign matches the sign of the |
| first argument instead, and so returns ``-1e-100`` in this case. Which approach |
| is more appropriate depends on the application. |
| |
| .. [#] If x is very close to an exact integer multiple of y, it's possible for |
| ``x//y`` to be one larger than ``(x-x%y)//y`` due to rounding. In such |
| cases, Python returns the latter result, in order to preserve that |
| ``divmod(x,y)[0] * y + x % y`` be very close to ``x``. |
| |
| .. [#] The Unicode standard distinguishes between :dfn:`code points` |
| (e.g. U+0041) and :dfn:`abstract characters` (e.g. "LATIN CAPITAL LETTER A"). |
| While most abstract characters in Unicode are only represented using one |
| code point, there is a number of abstract characters that can in addition be |
| represented using a sequence of more than one code point. For example, the |
| abstract character "LATIN CAPITAL LETTER C WITH CEDILLA" can be represented |
| as a single :dfn:`precomposed character` at code position U+00C7, or as a |
| sequence of a :dfn:`base character` at code position U+0043 (LATIN CAPITAL |
| LETTER C), followed by a :dfn:`combining character` at code position U+0327 |
| (COMBINING CEDILLA). |
| |
| The comparison operators on strings compare at the level of Unicode code |
| points. This may be counter-intuitive to humans. For example, |
| ``"\u00C7" == "\u0043\u0327"`` is ``False``, even though both strings |
| represent the same abstract character "LATIN CAPITAL LETTER C WITH CEDILLA". |
| |
| To compare strings at the level of abstract characters (that is, in a way |
| intuitive to humans), use :func:`unicodedata.normalize`. |
| |
| .. [#] Due to automatic garbage-collection, free lists, and the dynamic nature of |
| descriptors, you may notice seemingly unusual behaviour in certain uses of |
| the :keyword:`is` operator, like those involving comparisons between instance |
| methods, or constants. Check their documentation for more info. |
| |
| .. [#] The ``%`` operator is also used for string formatting; the same |
| precedence applies. |
| |
| .. [#] The power operator ``**`` binds less tightly than an arithmetic or |
| bitwise unary operator on its right, that is, ``2**-1`` is ``0.5``. |