Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 1 | .. _glossary: |
| 2 | |
| 3 | ******** |
| 4 | Glossary |
| 5 | ******** |
| 6 | |
| 7 | .. if you add new entries, keep the alphabetical sorting! |
| 8 | |
| 9 | .. glossary:: |
| 10 | |
| 11 | ``>>>`` |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 12 | The default Python prompt of the interactive shell. Often seen for code |
| 13 | examples which can be executed interactively in the interpreter. |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 14 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 15 | ``...`` |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 16 | The default Python prompt of the interactive shell when entering code for |
| 17 | an indented code block or within a pair of matching left and right |
| 18 | delimiters (parentheses, square brackets or curly braces). |
Christian Heimes | d8654cf | 2007-12-02 15:22:16 +0000 | [diff] [blame] | 19 | |
Benjamin Peterson | d631371 | 2008-07-31 16:23:04 +0000 | [diff] [blame] | 20 | 2to3 |
| 21 | A tool that tries to convert Python 2.x code to Python 3.x code by |
Georg Brandl | 6faee4e | 2010-09-21 14:48:28 +0000 | [diff] [blame] | 22 | handling most of the incompatibilities which can be detected by parsing the |
Benjamin Peterson | d631371 | 2008-07-31 16:23:04 +0000 | [diff] [blame] | 23 | source and traversing the parse tree. |
| 24 | |
| 25 | 2to3 is available in the standard library as :mod:`lib2to3`; a standalone |
| 26 | entry point is provided as :file:`Tools/scripts/2to3`. See |
| 27 | :ref:`2to3-reference`. |
| 28 | |
Georg Brandl | 86b2fb9 | 2008-07-16 03:43:04 +0000 | [diff] [blame] | 29 | abstract base class |
Éric Araujo | fa088db | 2011-06-04 18:42:38 +0200 | [diff] [blame] | 30 | Abstract base classes complement :term:`duck-typing` by |
Georg Brandl | 22b3431 | 2009-07-26 14:54:51 +0000 | [diff] [blame] | 31 | providing a way to define interfaces when other techniques like |
Éric Araujo | fa088db | 2011-06-04 18:42:38 +0200 | [diff] [blame] | 32 | :func:`hasattr` would be clumsy or subtly wrong (for example with |
Éric Araujo | 04ac59a | 2011-08-19 09:07:46 +0200 | [diff] [blame] | 33 | :ref:`magic methods <special-lookup>`). ABCs introduce virtual |
| 34 | subclasses, which are classes that don't inherit from a class but are |
| 35 | still recognized by :func:`isinstance` and :func:`issubclass`; see the |
| 36 | :mod:`abc` module documentation. Python comes with many built-in ABCs for |
Georg Brandl | 22b3431 | 2009-07-26 14:54:51 +0000 | [diff] [blame] | 37 | data structures (in the :mod:`collections` module), numbers (in the |
Éric Araujo | fa088db | 2011-06-04 18:42:38 +0200 | [diff] [blame] | 38 | :mod:`numbers` module), streams (in the :mod:`io` module), import finders |
| 39 | and loaders (in the :mod:`importlib.abc` module). You can create your own |
| 40 | ABCs with the :mod:`abc` module. |
Benjamin Peterson | 4118174 | 2008-07-02 20:22:54 +0000 | [diff] [blame] | 41 | |
Christian Heimes | d8654cf | 2007-12-02 15:22:16 +0000 | [diff] [blame] | 42 | argument |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 43 | A value passed to a function or method, assigned to a named local |
| 44 | variable in the function body. A function or method may have both |
| 45 | positional arguments and keyword arguments in its definition. |
| 46 | Positional and keyword arguments may be variable-length: ``*`` accepts |
| 47 | or passes (if in the function definition or call) several positional |
| 48 | arguments in a list, while ``**`` does the same for keyword arguments |
| 49 | in a dictionary. |
Christian Heimes | d8654cf | 2007-12-02 15:22:16 +0000 | [diff] [blame] | 50 | |
| 51 | Any expression may be used within the argument list, and the evaluated |
| 52 | value is passed to the local variable. |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 53 | |
| 54 | attribute |
| 55 | A value associated with an object which is referenced by name using |
| 56 | dotted expressions. For example, if an object *o* has an attribute |
| 57 | *a* it would be referenced as *o.a*. |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 58 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 59 | BDFL |
| 60 | Benevolent Dictator For Life, a.k.a. `Guido van Rossum |
| 61 | <http://www.python.org/~guido/>`_, Python's creator. |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 62 | |
Georg Brandl | 9afde1c | 2007-11-01 20:32:30 +0000 | [diff] [blame] | 63 | bytecode |
| 64 | Python source code is compiled into bytecode, the internal representation |
Brett Cannon | 8315fd1 | 2010-07-02 22:03:00 +0000 | [diff] [blame] | 65 | of a Python program in the CPython interpreter. The bytecode is also |
| 66 | cached in ``.pyc`` and ``.pyo`` files so that executing the same file is |
| 67 | faster the second time (recompilation from source to bytecode can be |
| 68 | avoided). This "intermediate language" is said to run on a |
| 69 | :term:`virtual machine` that executes the machine code corresponding to |
| 70 | each bytecode. Do note that bytecodes are not expected to work between |
| 71 | different Python virtual machines, nor to be stable between Python |
| 72 | releases. |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 73 | |
Georg Brandl | 2cb72d3 | 2010-07-03 10:26:54 +0000 | [diff] [blame] | 74 | A list of bytecode instructions can be found in the documentation for |
| 75 | :ref:`the dis module <bytecodes>`. |
| 76 | |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 77 | class |
| 78 | A template for creating user-defined objects. Class definitions |
| 79 | normally contain method definitions which operate on instances of the |
| 80 | class. |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 81 | |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 82 | coercion |
| 83 | The implicit conversion of an instance of one type to another during an |
| 84 | operation which involves two arguments of the same type. For example, |
| 85 | ``int(3.15)`` converts the floating point number to the integer ``3``, but |
| 86 | in ``3+4.5``, each argument is of a different type (one int, one float), |
| 87 | and both must be converted to the same type before they can be added or it |
Benjamin Peterson | 2d71822 | 2008-11-21 00:25:02 +0000 | [diff] [blame] | 88 | will raise a ``TypeError``. Without coercion, all arguments of even |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 89 | compatible types would have to be normalized to the same value by the |
| 90 | programmer, e.g., ``float(3)+4.5`` rather than just ``3+4.5``. |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 91 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 92 | complex number |
| 93 | An extension of the familiar real number system in which all numbers are |
| 94 | expressed as a sum of a real part and an imaginary part. Imaginary |
| 95 | numbers are real multiples of the imaginary unit (the square root of |
| 96 | ``-1``), often written ``i`` in mathematics or ``j`` in |
Georg Brandl | 22b3431 | 2009-07-26 14:54:51 +0000 | [diff] [blame] | 97 | engineering. Python has built-in support for complex numbers, which are |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 98 | written with this latter notation; the imaginary part is written with a |
| 99 | ``j`` suffix, e.g., ``3+1j``. To get access to complex equivalents of the |
| 100 | :mod:`math` module, use :mod:`cmath`. Use of complex numbers is a fairly |
| 101 | advanced mathematical feature. If you're not aware of a need for them, |
| 102 | it's almost certain you can safely ignore them. |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 103 | |
Christian Heimes | 895627f | 2007-12-08 17:28:33 +0000 | [diff] [blame] | 104 | context manager |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 105 | An object which controls the environment seen in a :keyword:`with` |
Christian Heimes | 895627f | 2007-12-08 17:28:33 +0000 | [diff] [blame] | 106 | statement by defining :meth:`__enter__` and :meth:`__exit__` methods. |
| 107 | See :pep:`343`. |
| 108 | |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 109 | CPython |
Antoine Pitrou | 0034281 | 2011-01-06 16:31:28 +0000 | [diff] [blame] | 110 | The canonical implementation of the Python programming language, as |
| 111 | distributed on `python.org <http://python.org>`_. The term "CPython" |
| 112 | is used when necessary to distinguish this implementation from others |
| 113 | such as Jython or IronPython. |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 114 | |
Christian Heimes | d8654cf | 2007-12-02 15:22:16 +0000 | [diff] [blame] | 115 | decorator |
| 116 | A function returning another function, usually applied as a function |
| 117 | transformation using the ``@wrapper`` syntax. Common examples for |
| 118 | decorators are :func:`classmethod` and :func:`staticmethod`. |
| 119 | |
| 120 | The decorator syntax is merely syntactic sugar, the following two |
| 121 | function definitions are semantically equivalent:: |
| 122 | |
| 123 | def f(...): |
| 124 | ... |
| 125 | f = staticmethod(f) |
| 126 | |
| 127 | @staticmethod |
| 128 | def f(...): |
| 129 | ... |
| 130 | |
Georg Brandl | af265f4 | 2008-12-07 15:06:20 +0000 | [diff] [blame] | 131 | The same concept exists for classes, but is less commonly used there. See |
| 132 | the documentation for :ref:`function definitions <function>` and |
| 133 | :ref:`class definitions <class>` for more about decorators. |
Georg Brandl | a09ca38 | 2007-12-02 18:20:12 +0000 | [diff] [blame] | 134 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 135 | descriptor |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 136 | Any object which defines the methods :meth:`__get__`, :meth:`__set__`, or |
Georg Brandl | 85eb8c1 | 2007-08-31 16:33:38 +0000 | [diff] [blame] | 137 | :meth:`__delete__`. When a class attribute is a descriptor, its special |
Georg Brandl | 9afde1c | 2007-11-01 20:32:30 +0000 | [diff] [blame] | 138 | binding behavior is triggered upon attribute lookup. Normally, using |
| 139 | *a.b* to get, set or delete an attribute looks up the object named *b* in |
| 140 | the class dictionary for *a*, but if *b* is a descriptor, the respective |
| 141 | descriptor method gets called. Understanding descriptors is a key to a |
| 142 | deep understanding of Python because they are the basis for many features |
| 143 | including functions, methods, properties, class methods, static methods, |
| 144 | and reference to super classes. |
| 145 | |
| 146 | For more information about descriptors' methods, see :ref:`descriptors`. |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 147 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 148 | dictionary |
Senthil Kumaran | 6080db7 | 2012-03-12 10:05:34 -0700 | [diff] [blame] | 149 | An associative array, where arbitrary keys are mapped to values. The |
| 150 | keys can be any object with :meth:`__hash__` and :meth:`__eq__` methods. |
| 151 | Called a hash in Perl. |
Georg Brandl | 3dbca81 | 2008-07-23 16:10:53 +0000 | [diff] [blame] | 152 | |
| 153 | docstring |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 154 | A string literal which appears as the first expression in a class, |
| 155 | function or module. While ignored when the suite is executed, it is |
| 156 | recognized by the compiler and put into the :attr:`__doc__` attribute |
| 157 | of the enclosing class, function or module. Since it is available via |
| 158 | introspection, it is the canonical place for documentation of the |
Georg Brandl | 3dbca81 | 2008-07-23 16:10:53 +0000 | [diff] [blame] | 159 | object. |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 160 | |
| 161 | duck-typing |
Georg Brandl | 73b1c7b | 2010-07-10 10:39:57 +0000 | [diff] [blame] | 162 | A programming style which does not look at an object's type to determine |
| 163 | if it has the right interface; instead, the method or attribute is simply |
| 164 | called or used ("If it looks like a duck and quacks like a duck, it |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 165 | must be a duck.") By emphasizing interfaces rather than specific types, |
| 166 | well-designed code improves its flexibility by allowing polymorphic |
| 167 | substitution. Duck-typing avoids tests using :func:`type` or |
Georg Brandl | 8a1c254 | 2010-07-11 08:36:20 +0000 | [diff] [blame] | 168 | :func:`isinstance`. (Note, however, that duck-typing can be complemented |
Éric Araujo | 0519b09 | 2011-08-19 00:39:19 +0200 | [diff] [blame] | 169 | with :term:`abstract base classes <abstract base class>`.) Instead, it |
| 170 | typically employs :func:`hasattr` tests or :term:`EAFP` programming. |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 171 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 172 | EAFP |
| 173 | Easier to ask for forgiveness than permission. This common Python coding |
| 174 | style assumes the existence of valid keys or attributes and catches |
| 175 | exceptions if the assumption proves false. This clean and fast style is |
| 176 | characterized by the presence of many :keyword:`try` and :keyword:`except` |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 177 | statements. The technique contrasts with the :term:`LBYL` style |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 178 | common to many other languages such as C. |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 179 | |
Christian Heimes | d8654cf | 2007-12-02 15:22:16 +0000 | [diff] [blame] | 180 | expression |
| 181 | A piece of syntax which can be evaluated to some value. In other words, |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 182 | an expression is an accumulation of expression elements like literals, |
| 183 | names, attribute access, operators or function calls which all return a |
| 184 | value. In contrast to many other languages, not all language constructs |
| 185 | are expressions. There are also :term:`statement`\s which cannot be used |
| 186 | as expressions, such as :keyword:`if`. Assignments are also statements, |
| 187 | not expressions. |
Christian Heimes | d8654cf | 2007-12-02 15:22:16 +0000 | [diff] [blame] | 188 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 189 | extension module |
Georg Brandl | 9d9848e | 2010-12-28 11:48:53 +0000 | [diff] [blame] | 190 | A module written in C or C++, using Python's C API to interact with the |
| 191 | core and with user code. |
Christian Heimes | d8654cf | 2007-12-02 15:22:16 +0000 | [diff] [blame] | 192 | |
Antoine Pitrou | 0b65b0f | 2010-09-15 09:58:26 +0000 | [diff] [blame] | 193 | file object |
| 194 | An object exposing a file-oriented API (with methods such as |
Georg Brandl | 9d9848e | 2010-12-28 11:48:53 +0000 | [diff] [blame] | 195 | :meth:`read()` or :meth:`write()`) to an underlying resource. Depending |
| 196 | on the way it was created, a file object can mediate access to a real |
Sandro Tosi | 7f67133 | 2012-06-02 23:41:19 +0200 | [diff] [blame] | 197 | on-disk file or to another type of storage or communication device |
Georg Brandl | 9d9848e | 2010-12-28 11:48:53 +0000 | [diff] [blame] | 198 | (for example standard input/output, in-memory buffers, sockets, pipes, |
| 199 | etc.). File objects are also called :dfn:`file-like objects` or |
| 200 | :dfn:`streams`. |
Antoine Pitrou | 0b65b0f | 2010-09-15 09:58:26 +0000 | [diff] [blame] | 201 | |
Georg Brandl | 9d9848e | 2010-12-28 11:48:53 +0000 | [diff] [blame] | 202 | There are actually three categories of file objects: raw binary files, |
| 203 | buffered binary files and text files. Their interfaces are defined in the |
| 204 | :mod:`io` module. The canonical way to create a file object is by using |
| 205 | the :func:`open` function. |
Antoine Pitrou | 0b65b0f | 2010-09-15 09:58:26 +0000 | [diff] [blame] | 206 | |
| 207 | file-like object |
| 208 | A synonym for :term:`file object`. |
| 209 | |
Brett Cannon | 51d4aab | 2009-01-25 04:21:39 +0000 | [diff] [blame] | 210 | finder |
| 211 | An object that tries to find the :term:`loader` for a module. It must |
| 212 | implement a method named :meth:`find_module`. See :pep:`302` for |
Brett Cannon | e43b060 | 2009-03-21 03:11:16 +0000 | [diff] [blame] | 213 | details and :class:`importlib.abc.Finder` for an |
| 214 | :term:`abstract base class`. |
Brett Cannon | 51d4aab | 2009-01-25 04:21:39 +0000 | [diff] [blame] | 215 | |
Benjamin Peterson | 2d71822 | 2008-11-21 00:25:02 +0000 | [diff] [blame] | 216 | floor division |
Raymond Hettinger | f37ca3c | 2010-09-01 22:11:53 +0000 | [diff] [blame] | 217 | Mathematical division that rounds down to nearest integer. The floor |
| 218 | division operator is ``//``. For example, the expression ``11 // 4`` |
| 219 | evaluates to ``2`` in contrast to the ``2.75`` returned by float true |
| 220 | division. Note that ``(-11) // 4`` is ``-3`` because that is ``-2.75`` |
| 221 | rounded *downward*. See :pep:`238`. |
Benjamin Peterson | 2d71822 | 2008-11-21 00:25:02 +0000 | [diff] [blame] | 222 | |
Christian Heimes | d8654cf | 2007-12-02 15:22:16 +0000 | [diff] [blame] | 223 | function |
| 224 | A series of statements which returns some value to a caller. It can also |
| 225 | be passed zero or more arguments which may be used in the execution of |
| 226 | the body. See also :term:`argument` and :term:`method`. |
| 227 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 228 | __future__ |
Raymond Hettinger | f37ca3c | 2010-09-01 22:11:53 +0000 | [diff] [blame] | 229 | A pseudo-module which programmers can use to enable new language features |
Benjamin Peterson | 2d71822 | 2008-11-21 00:25:02 +0000 | [diff] [blame] | 230 | which are not compatible with the current interpreter. |
| 231 | |
| 232 | By importing the :mod:`__future__` module and evaluating its variables, |
| 233 | you can see when a new feature was first added to the language and when it |
| 234 | becomes the default:: |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 235 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 236 | >>> import __future__ |
| 237 | >>> __future__.division |
| 238 | _Feature((2, 2, 0, 'alpha', 2), (3, 0, 0, 'alpha', 0), 8192) |
| 239 | |
| 240 | garbage collection |
| 241 | The process of freeing memory when it is not used anymore. Python |
| 242 | performs garbage collection via reference counting and a cyclic garbage |
| 243 | collector that is able to detect and break reference cycles. |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 244 | |
Benjamin Peterson | 08bf91c | 2010-04-11 16:12:57 +0000 | [diff] [blame] | 245 | .. index:: single: generator |
| 246 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 247 | generator |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 248 | A function which returns an iterator. It looks like a normal function |
Raymond Hettinger | f37ca3c | 2010-09-01 22:11:53 +0000 | [diff] [blame] | 249 | except that it contains :keyword:`yield` statements for producing a series |
| 250 | a values usable in a for-loop or that can be retrieved one at a time with |
| 251 | the :func:`next` function. Each :keyword:`yield` temporarily suspends |
| 252 | processing, remembering the location execution state (including local |
| 253 | variables and pending try-statements). When the generator resumes, it |
| 254 | picks-up where it left-off (in contrast to functions which start fresh on |
Éric Araujo | e0854f9 | 2011-05-27 04:36:52 +0200 | [diff] [blame] | 255 | every invocation). |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 256 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 257 | .. index:: single: generator expression |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 258 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 259 | generator expression |
Benjamin Peterson | 08bf91c | 2010-04-11 16:12:57 +0000 | [diff] [blame] | 260 | An expression that returns an iterator. It looks like a normal expression |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 261 | followed by a :keyword:`for` expression defining a loop variable, range, |
| 262 | and an optional :keyword:`if` expression. The combined expression |
| 263 | generates values for an enclosing function:: |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 264 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 265 | >>> sum(i*i for i in range(10)) # sum of squares 0, 1, 4, ... 81 |
| 266 | 285 |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 267 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 268 | GIL |
| 269 | See :term:`global interpreter lock`. |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 270 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 271 | global interpreter lock |
Antoine Pitrou | 0034281 | 2011-01-06 16:31:28 +0000 | [diff] [blame] | 272 | The mechanism used by the :term:`CPython` interpreter to assure that |
| 273 | only one thread executes Python :term:`bytecode` at a time. |
| 274 | This simplifies the CPython implementation by making the object model |
| 275 | (including critical built-in types such as :class:`dict`) implicitly |
| 276 | safe against concurrent access. Locking the entire interpreter |
| 277 | makes it easier for the interpreter to be multi-threaded, at the |
| 278 | expense of much of the parallelism afforded by multi-processor |
| 279 | machines. |
| 280 | |
| 281 | However, some extension modules, either standard or third-party, |
| 282 | are designed so as to release the GIL when doing computationally-intensive |
| 283 | tasks such as compression or hashing. Also, the GIL is always released |
| 284 | when doing I/O. |
| 285 | |
| 286 | Past efforts to create a "free-threaded" interpreter (one which locks |
| 287 | shared data at a much finer granularity) have not been successful |
| 288 | because performance suffered in the common single-processor case. It |
| 289 | is believed that overcoming this performance issue would make the |
| 290 | implementation much more complicated and therefore costlier to maintain. |
Guido van Rossum | 2cc30da | 2007-11-02 23:46:40 +0000 | [diff] [blame] | 291 | |
| 292 | hashable |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 293 | An object is *hashable* if it has a hash value which never changes during |
Guido van Rossum | 2cc30da | 2007-11-02 23:46:40 +0000 | [diff] [blame] | 294 | its lifetime (it needs a :meth:`__hash__` method), and can be compared to |
Georg Brandl | 05f5ab7 | 2008-09-24 09:11:47 +0000 | [diff] [blame] | 295 | other objects (it needs an :meth:`__eq__` method). Hashable objects which |
| 296 | compare equal must have the same hash value. |
Guido van Rossum | 2cc30da | 2007-11-02 23:46:40 +0000 | [diff] [blame] | 297 | |
| 298 | Hashability makes an object usable as a dictionary key and a set member, |
| 299 | because these data structures use the hash value internally. |
| 300 | |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 301 | All of Python's immutable built-in objects are hashable, while no mutable |
| 302 | containers (such as lists or dictionaries) are. Objects which are |
Guido van Rossum | 2cc30da | 2007-11-02 23:46:40 +0000 | [diff] [blame] | 303 | instances of user-defined classes are hashable by default; they all |
| 304 | compare unequal, and their hash value is their :func:`id`. |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 305 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 306 | IDLE |
| 307 | An Integrated Development Environment for Python. IDLE is a basic editor |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 308 | and interpreter environment which ships with the standard distribution of |
Raymond Hettinger | f37ca3c | 2010-09-01 22:11:53 +0000 | [diff] [blame] | 309 | Python. |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 310 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 311 | immutable |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 312 | An object with a fixed value. Immutable objects include numbers, strings and |
| 313 | tuples. Such an object cannot be altered. A new object has to |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 314 | be created if a different value has to be stored. They play an important |
| 315 | role in places where a constant hash value is needed, for example as a key |
| 316 | in a dictionary. |
Benjamin Peterson | 2d71822 | 2008-11-21 00:25:02 +0000 | [diff] [blame] | 317 | |
Brett Cannon | 51d4aab | 2009-01-25 04:21:39 +0000 | [diff] [blame] | 318 | importer |
| 319 | An object that both finds and loads a module; both a |
| 320 | :term:`finder` and :term:`loader` object. |
| 321 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 322 | interactive |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 323 | Python has an interactive interpreter which means you can enter |
| 324 | statements and expressions at the interpreter prompt, immediately |
| 325 | execute them and see their results. Just launch ``python`` with no |
| 326 | arguments (possibly by selecting it from your computer's main |
| 327 | menu). It is a very powerful way to test out new ideas or inspect |
| 328 | modules and packages (remember ``help(x)``). |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 329 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 330 | interpreted |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 331 | Python is an interpreted language, as opposed to a compiled one, |
| 332 | though the distinction can be blurry because of the presence of the |
| 333 | bytecode compiler. This means that source files can be run directly |
| 334 | without explicitly creating an executable which is then run. |
| 335 | Interpreted languages typically have a shorter development/debug cycle |
| 336 | than compiled ones, though their programs generally also run more |
| 337 | slowly. See also :term:`interactive`. |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 338 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 339 | iterable |
Raymond Hettinger | b6b76c2 | 2010-11-14 05:27:15 +0000 | [diff] [blame] | 340 | An object capable of returning its members one at a |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 341 | time. Examples of iterables include all sequence types (such as |
| 342 | :class:`list`, :class:`str`, and :class:`tuple`) and some non-sequence |
| 343 | types like :class:`dict` and :class:`file` and objects of any classes you |
| 344 | define with an :meth:`__iter__` or :meth:`__getitem__` method. Iterables |
| 345 | can be used in a :keyword:`for` loop and in many other places where a |
| 346 | sequence is needed (:func:`zip`, :func:`map`, ...). When an iterable |
Georg Brandl | 22b3431 | 2009-07-26 14:54:51 +0000 | [diff] [blame] | 347 | object is passed as an argument to the built-in function :func:`iter`, it |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 348 | returns an iterator for the object. This iterator is good for one pass |
| 349 | over the set of values. When using iterables, it is usually not necessary |
| 350 | to call :func:`iter` or deal with iterator objects yourself. The ``for`` |
| 351 | statement does that automatically for you, creating a temporary unnamed |
| 352 | variable to hold the iterator for the duration of the loop. See also |
| 353 | :term:`iterator`, :term:`sequence`, and :term:`generator`. |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 354 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 355 | iterator |
| 356 | An object representing a stream of data. Repeated calls to the iterator's |
Ezio Melotti | 7fa8222 | 2012-10-12 13:42:08 +0300 | [diff] [blame] | 357 | :meth:`~iterator.__next__` method (or passing it to the built-in function |
Georg Brandl | b30f330 | 2011-01-06 09:23:56 +0000 | [diff] [blame] | 358 | :func:`next`) return successive items in the stream. When no more data |
| 359 | are available a :exc:`StopIteration` exception is raised instead. At this |
Benjamin Peterson | e7c78b2 | 2008-07-03 20:28:26 +0000 | [diff] [blame] | 360 | point, the iterator object is exhausted and any further calls to its |
Georg Brandl | b30f330 | 2011-01-06 09:23:56 +0000 | [diff] [blame] | 361 | :meth:`__next__` method just raise :exc:`StopIteration` again. Iterators |
| 362 | are required to have an :meth:`__iter__` method that returns the iterator |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 363 | object itself so every iterator is also iterable and may be used in most |
| 364 | places where other iterables are accepted. One notable exception is code |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 365 | which attempts multiple iteration passes. A container object (such as a |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 366 | :class:`list`) produces a fresh new iterator each time you pass it to the |
| 367 | :func:`iter` function or use it in a :keyword:`for` loop. Attempting this |
| 368 | with an iterator will just return the same exhausted iterator object used |
| 369 | in the previous iteration pass, making it appear like an empty container. |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 370 | |
Georg Brandl | 9afde1c | 2007-11-01 20:32:30 +0000 | [diff] [blame] | 371 | More information can be found in :ref:`typeiter`. |
| 372 | |
Georg Brandl | c275e15 | 2010-11-05 07:10:41 +0000 | [diff] [blame] | 373 | key function |
| 374 | A key function or collation function is a callable that returns a value |
| 375 | used for sorting or ordering. For example, :func:`locale.strxfrm` is |
| 376 | used to produce a sort key that is aware of locale specific sort |
| 377 | conventions. |
| 378 | |
| 379 | A number of tools in Python accept key functions to control how elements |
| 380 | are ordered or grouped. They include :func:`min`, :func:`max`, |
| 381 | :func:`sorted`, :meth:`list.sort`, :func:`heapq.nsmallest`, |
| 382 | :func:`heapq.nlargest`, and :func:`itertools.groupby`. |
| 383 | |
| 384 | There are several ways to create a key function. For example. the |
| 385 | :meth:`str.lower` method can serve as a key function for case insensitive |
| 386 | sorts. Alternatively, an ad-hoc key function can be built from a |
| 387 | :keyword:`lambda` expression such as ``lambda r: (r[0], r[2])``. Also, |
Sandro Tosi | 165a2c2 | 2012-04-01 01:50:00 +0200 | [diff] [blame] | 388 | the :mod:`operator` module provides three key function constructors: |
Georg Brandl | c275e15 | 2010-11-05 07:10:41 +0000 | [diff] [blame] | 389 | :func:`~operator.attrgetter`, :func:`~operator.itemgetter`, and |
| 390 | :func:`~operator.methodcaller`. See the :ref:`Sorting HOW TO |
| 391 | <sortinghowto>` for examples of how to create and use key functions. |
| 392 | |
Christian Heimes | d8654cf | 2007-12-02 15:22:16 +0000 | [diff] [blame] | 393 | keyword argument |
| 394 | Arguments which are preceded with a ``variable_name=`` in the call. |
| 395 | The variable name designates the local name in the function to which the |
| 396 | value is assigned. ``**`` is used to accept or pass a dictionary of |
| 397 | keyword arguments. See :term:`argument`. |
| 398 | |
| 399 | lambda |
| 400 | An anonymous inline function consisting of a single :term:`expression` |
| 401 | which is evaluated when the function is called. The syntax to create |
| 402 | a lambda function is ``lambda [arguments]: expression`` |
| 403 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 404 | LBYL |
| 405 | Look before you leap. This coding style explicitly tests for |
| 406 | pre-conditions before making calls or lookups. This style contrasts with |
| 407 | the :term:`EAFP` approach and is characterized by the presence of many |
| 408 | :keyword:`if` statements. |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 409 | |
Raymond Hettinger | 09f4414 | 2010-12-17 20:19:50 +0000 | [diff] [blame] | 410 | In a multi-threaded environment, the LBYL approach can risk introducing a |
| 411 | race condition between "the looking" and "the leaping". For example, the |
| 412 | code, ``if key in mapping: return mapping[key]`` can fail if another |
| 413 | thread removes *key* from *mapping* after the test, but before the lookup. |
| 414 | This issue can be solved with locks or by using the EAFP approach. |
| 415 | |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 416 | list |
| 417 | A built-in Python :term:`sequence`. Despite its name it is more akin |
| 418 | to an array in other languages than to a linked list since access to |
| 419 | elements are O(1). |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 420 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 421 | list comprehension |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 422 | A compact way to process all or part of the elements in a sequence and |
Georg Brandl | ede6c2a | 2010-01-05 10:22:04 +0000 | [diff] [blame] | 423 | return a list with the results. ``result = ['{:#04x}'.format(x) for x in |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 424 | range(256) if x % 2 == 0]`` generates a list of strings containing |
| 425 | even hex numbers (0x..) in the range from 0 to 255. The :keyword:`if` |
| 426 | clause is optional. If omitted, all elements in ``range(256)`` are |
| 427 | processed. |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 428 | |
Brett Cannon | 51d4aab | 2009-01-25 04:21:39 +0000 | [diff] [blame] | 429 | loader |
| 430 | An object that loads a module. It must define a method named |
| 431 | :meth:`load_module`. A loader is typically returned by a |
Brett Cannon | e43b060 | 2009-03-21 03:11:16 +0000 | [diff] [blame] | 432 | :term:`finder`. See :pep:`302` for details and |
| 433 | :class:`importlib.abc.Loader` for an :term:`abstract base class`. |
Brett Cannon | 51d4aab | 2009-01-25 04:21:39 +0000 | [diff] [blame] | 434 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 435 | mapping |
Raymond Hettinger | e3ee66f | 2011-01-08 23:44:37 +0000 | [diff] [blame] | 436 | A container object that supports arbitrary key lookups and implements the |
Éric Araujo | 7c12bae | 2011-07-26 15:14:35 +0200 | [diff] [blame] | 437 | methods specified in the :class:`~collections.Mapping` or |
| 438 | :class:`~collections.MutableMapping` |
Éric Araujo | fa088db | 2011-06-04 18:42:38 +0200 | [diff] [blame] | 439 | :ref:`abstract base classes <collections-abstract-base-classes>`. Examples |
| 440 | include :class:`dict`, :class:`collections.defaultdict`, |
Raymond Hettinger | e3ee66f | 2011-01-08 23:44:37 +0000 | [diff] [blame] | 441 | :class:`collections.OrderedDict` and :class:`collections.Counter`. |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 442 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 443 | metaclass |
| 444 | The class of a class. Class definitions create a class name, a class |
| 445 | dictionary, and a list of base classes. The metaclass is responsible for |
| 446 | taking those three arguments and creating the class. Most object oriented |
| 447 | programming languages provide a default implementation. What makes Python |
| 448 | special is that it is possible to create custom metaclasses. Most users |
| 449 | never need this tool, but when the need arises, metaclasses can provide |
| 450 | powerful, elegant solutions. They have been used for logging attribute |
| 451 | access, adding thread-safety, tracking object creation, implementing |
| 452 | singletons, and many other tasks. |
Georg Brandl | 9afde1c | 2007-11-01 20:32:30 +0000 | [diff] [blame] | 453 | |
| 454 | More information can be found in :ref:`metaclasses`. |
Christian Heimes | d8654cf | 2007-12-02 15:22:16 +0000 | [diff] [blame] | 455 | |
| 456 | method |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 457 | A function which is defined inside a class body. If called as an attribute |
Christian Heimes | d8654cf | 2007-12-02 15:22:16 +0000 | [diff] [blame] | 458 | of an instance of that class, the method will get the instance object as |
| 459 | its first :term:`argument` (which is usually called ``self``). |
| 460 | See :term:`function` and :term:`nested scope`. |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 461 | |
Michael Foord | 95fc51d | 2010-11-20 15:07:30 +0000 | [diff] [blame] | 462 | method resolution order |
| 463 | Method Resolution Order is the order in which base classes are searched |
| 464 | for a member during lookup. See `The Python 2.3 Method Resolution Order |
| 465 | <http://www.python.org/download/releases/2.3/mro/>`_. |
| 466 | |
| 467 | MRO |
| 468 | See :term:`method resolution order`. |
| 469 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 470 | mutable |
| 471 | Mutable objects can change their value but keep their :func:`id`. See |
| 472 | also :term:`immutable`. |
Christian Heimes | 25bb783 | 2008-01-11 16:17:00 +0000 | [diff] [blame] | 473 | |
| 474 | named tuple |
Raymond Hettinger | d04fa31 | 2009-02-04 19:45:13 +0000 | [diff] [blame] | 475 | Any tuple-like class whose indexable elements are also accessible using |
Christian Heimes | d32ed6f | 2008-01-14 18:49:24 +0000 | [diff] [blame] | 476 | named attributes (for example, :func:`time.localtime` returns a |
| 477 | tuple-like object where the *year* is accessible either with an |
| 478 | index such as ``t[0]`` or with a named attribute like ``t.tm_year``). |
| 479 | |
| 480 | A named tuple can be a built-in type such as :class:`time.struct_time`, |
| 481 | or it can be created with a regular class definition. A full featured |
| 482 | named tuple can also be created with the factory function |
| 483 | :func:`collections.namedtuple`. The latter approach automatically |
| 484 | provides extra features such as a self-documenting representation like |
| 485 | ``Employee(name='jones', title='programmer')``. |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 486 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 487 | namespace |
| 488 | The place where a variable is stored. Namespaces are implemented as |
Georg Brandl | 22b3431 | 2009-07-26 14:54:51 +0000 | [diff] [blame] | 489 | dictionaries. There are the local, global and built-in namespaces as well |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 490 | as nested namespaces in objects (in methods). Namespaces support |
| 491 | modularity by preventing naming conflicts. For instance, the functions |
Georg Brandl | 1a3284e | 2007-12-02 09:40:06 +0000 | [diff] [blame] | 492 | :func:`builtins.open` and :func:`os.open` are distinguished by their |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 493 | namespaces. Namespaces also aid readability and maintainability by making |
| 494 | it clear which module implements a function. For instance, writing |
Éric Araujo | 7af8ebb | 2011-09-01 03:20:13 +0200 | [diff] [blame] | 495 | :func:`random.seed` or :func:`itertools.islice` makes it clear that those |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 496 | functions are implemented by the :mod:`random` and :mod:`itertools` |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 497 | modules, respectively. |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 498 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 499 | nested scope |
| 500 | The ability to refer to a variable in an enclosing definition. For |
| 501 | instance, a function defined inside another function can refer to |
Benjamin Peterson | 927ccd2 | 2010-06-29 18:36:39 +0000 | [diff] [blame] | 502 | variables in the outer function. Note that nested scopes by default work |
| 503 | only for reference and not for assignment. Local variables both read and |
| 504 | write in the innermost scope. Likewise, global variables read and write |
| 505 | to the global namespace. The :keyword:`nonlocal` allows writing to outer |
| 506 | scopes. |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 507 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 508 | new-style class |
Georg Brandl | 85eb8c1 | 2007-08-31 16:33:38 +0000 | [diff] [blame] | 509 | Old name for the flavor of classes now used for all class objects. In |
| 510 | earlier Python versions, only new-style classes could use Python's newer, |
| 511 | versatile features like :attr:`__slots__`, descriptors, properties, |
| 512 | :meth:`__getattribute__`, class methods, and static methods. |
Georg Brandl | 9afde1c | 2007-11-01 20:32:30 +0000 | [diff] [blame] | 513 | |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 514 | object |
| 515 | Any data with state (attributes or value) and defined behavior |
| 516 | (methods). Also the ultimate base class of any :term:`new-style |
| 517 | class`. |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 518 | |
Christian Heimes | d8654cf | 2007-12-02 15:22:16 +0000 | [diff] [blame] | 519 | positional argument |
| 520 | The arguments assigned to local names inside a function or method, |
| 521 | determined by the order in which they were given in the call. ``*`` is |
| 522 | used to either accept multiple positional arguments (when in the |
| 523 | definition), or pass several arguments as a list to a function. See |
| 524 | :term:`argument`. |
| 525 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 526 | Python 3000 |
Benjamin Peterson | 1e2f050 | 2008-05-26 12:52:02 +0000 | [diff] [blame] | 527 | Nickname for the Python 3.x release line (coined long ago when the release |
| 528 | of version 3 was something in the distant future.) This is also |
| 529 | abbreviated "Py3k". |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 530 | |
Christian Heimes | d8654cf | 2007-12-02 15:22:16 +0000 | [diff] [blame] | 531 | Pythonic |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 532 | An idea or piece of code which closely follows the most common idioms |
| 533 | of the Python language, rather than implementing code using concepts |
| 534 | common to other languages. For example, a common idiom in Python is |
| 535 | to loop over all elements of an iterable using a :keyword:`for` |
| 536 | statement. Many other languages don't have this type of construct, so |
| 537 | people unfamiliar with Python sometimes use a numerical counter instead:: |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 538 | |
Christian Heimes | d8654cf | 2007-12-02 15:22:16 +0000 | [diff] [blame] | 539 | for i in range(len(food)): |
Georg Brandl | a09ca38 | 2007-12-02 18:20:12 +0000 | [diff] [blame] | 540 | print(food[i]) |
Christian Heimes | d8654cf | 2007-12-02 15:22:16 +0000 | [diff] [blame] | 541 | |
| 542 | As opposed to the cleaner, Pythonic method:: |
| 543 | |
| 544 | for piece in food: |
Georg Brandl | a09ca38 | 2007-12-02 18:20:12 +0000 | [diff] [blame] | 545 | print(piece) |
Christian Heimes | d8654cf | 2007-12-02 15:22:16 +0000 | [diff] [blame] | 546 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 547 | reference count |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 548 | The number of references to an object. When the reference count of an |
| 549 | object drops to zero, it is deallocated. Reference counting is |
| 550 | generally not visible to Python code, but it is a key element of the |
| 551 | :term:`CPython` implementation. The :mod:`sys` module defines a |
Georg Brandl | ede6c2a | 2010-01-05 10:22:04 +0000 | [diff] [blame] | 552 | :func:`~sys.getrefcount` function that programmers can call to return the |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 553 | reference count for a particular object. |
| 554 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 555 | __slots__ |
Georg Brandl | 85eb8c1 | 2007-08-31 16:33:38 +0000 | [diff] [blame] | 556 | A declaration inside a class that saves memory by pre-declaring space for |
| 557 | instance attributes and eliminating instance dictionaries. Though |
| 558 | popular, the technique is somewhat tricky to get right and is best |
| 559 | reserved for rare cases where there are large numbers of instances in a |
| 560 | memory-critical application. |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 561 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 562 | sequence |
| 563 | An :term:`iterable` which supports efficient element access using integer |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 564 | indices via the :meth:`__getitem__` special method and defines a |
Andrew Svetlov | 8cf1cc4 | 2012-10-05 13:26:10 +0300 | [diff] [blame] | 565 | :meth:`__len__` method that returns the length of the sequence. |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 566 | Some built-in sequence types are :class:`list`, :class:`str`, |
Georg Brandl | 2ae8ac2 | 2009-02-05 10:40:48 +0000 | [diff] [blame] | 567 | :class:`tuple`, and :class:`bytes`. Note that :class:`dict` also |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 568 | supports :meth:`__getitem__` and :meth:`__len__`, but is considered a |
| 569 | mapping rather than a sequence because the lookups use arbitrary |
| 570 | :term:`immutable` keys rather than integers. |
| 571 | |
Christian Heimes | d8654cf | 2007-12-02 15:22:16 +0000 | [diff] [blame] | 572 | slice |
Georg Brandl | c6fe37b | 2007-12-03 21:07:25 +0000 | [diff] [blame] | 573 | An object usually containing a portion of a :term:`sequence`. A slice is |
Christian Heimes | d8654cf | 2007-12-02 15:22:16 +0000 | [diff] [blame] | 574 | created using the subscript notation, ``[]`` with colons between numbers |
| 575 | when several are given, such as in ``variable_name[1:3:5]``. The bracket |
Georg Brandl | a09ca38 | 2007-12-02 18:20:12 +0000 | [diff] [blame] | 576 | (subscript) notation uses :class:`slice` objects internally. |
Christian Heimes | d8654cf | 2007-12-02 15:22:16 +0000 | [diff] [blame] | 577 | |
Georg Brandl | af265f4 | 2008-12-07 15:06:20 +0000 | [diff] [blame] | 578 | special method |
| 579 | A method that is called implicitly by Python to execute a certain |
| 580 | operation on a type, such as addition. Such methods have names starting |
| 581 | and ending with double underscores. Special methods are documented in |
| 582 | :ref:`specialnames`. |
| 583 | |
Christian Heimes | d8654cf | 2007-12-02 15:22:16 +0000 | [diff] [blame] | 584 | statement |
| 585 | A statement is part of a suite (a "block" of code). A statement is either |
| 586 | an :term:`expression` or a one of several constructs with a keyword, such |
Georg Brandl | a09ca38 | 2007-12-02 18:20:12 +0000 | [diff] [blame] | 587 | as :keyword:`if`, :keyword:`while` or :keyword:`for`. |
Christian Heimes | d8654cf | 2007-12-02 15:22:16 +0000 | [diff] [blame] | 588 | |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 589 | triple-quoted string |
| 590 | A string which is bound by three instances of either a quotation mark |
| 591 | (") or an apostrophe ('). While they don't provide any functionality |
| 592 | not available with single-quoted strings, they are useful for a number |
| 593 | of reasons. They allow you to include unescaped single and double |
| 594 | quotes within a string and they can span multiple lines without the |
| 595 | use of the continuation character, making them especially useful when |
| 596 | writing docstrings. |
| 597 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 598 | type |
| 599 | The type of a Python object determines what kind of object it is; every |
| 600 | object has a type. An object's type is accessible as its |
| 601 | :attr:`__class__` attribute or can be retrieved with ``type(obj)``. |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 602 | |
R David Murray | 1b00f25 | 2012-08-15 10:43:58 -0400 | [diff] [blame] | 603 | universal newlines |
| 604 | A manner of interpreting text streams in which all of the following are |
| 605 | recognized as ending a line: the Unix end-of-line convention ``'\n'``, |
| 606 | the Windows convention ``'\r\n'``, and the old Macintosh convention |
| 607 | ``'\r'``. See :pep:`278` and :pep:`3116`, as well as |
| 608 | :func:`str.splitlines` for an additional use. |
| 609 | |
Benjamin Peterson | 656aa28 | 2008-11-21 23:22:00 +0000 | [diff] [blame] | 610 | view |
Ezio Melotti | 619de8f | 2009-06-25 18:39:31 +0000 | [diff] [blame] | 611 | The objects returned from :meth:`dict.keys`, :meth:`dict.values`, and |
Benjamin Peterson | 656aa28 | 2008-11-21 23:22:00 +0000 | [diff] [blame] | 612 | :meth:`dict.items` are called dictionary views. They are lazy sequences |
| 613 | that will see changes in the underlying dictionary. To force the |
| 614 | dictionary view to become a full list use ``list(dictview)``. See |
| 615 | :ref:`dict-views`. |
| 616 | |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 617 | virtual machine |
| 618 | A computer defined entirely in software. Python's virtual machine |
| 619 | executes the :term:`bytecode` emitted by the bytecode compiler. |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 620 | |
Guido van Rossum | f10aa98 | 2007-08-17 18:30:38 +0000 | [diff] [blame] | 621 | Zen of Python |
| 622 | Listing of Python design principles and philosophies that are helpful in |
| 623 | understanding and using the language. The listing can be found by typing |
| 624 | "``import this``" at the interactive prompt. |