Make TextIOWrapper's seek/tell work properly with stateful decoders;
document and rename things to make seek/tell workings a little clearer.
Add a weird decoder for testing TextIOWrapper's seek/tell methods.
Document the getstate/setstate protocol conventions for IncrementalDecoders.
diff --git a/Lib/io.py b/Lib/io.py
index 98843d3..d3c9f85 100644
--- a/Lib/io.py
+++ b/Lib/io.py
@@ -802,11 +802,10 @@
return self._read_buf
def read1(self, n):
- """Reads up to n bytes.
+ """Reads up to n bytes, with at most one read() system call.
- Returns up to n bytes. If at least one byte is buffered,
- we only return buffered bytes. Otherwise, we do one
- raw read.
+ Returns up to n bytes. If at least one byte is buffered, we
+ only return buffered bytes. Otherwise, we do one raw read.
"""
if n <= 0:
return b""
@@ -1180,10 +1179,24 @@
self._writenl = newline or os.linesep
self._encoder = None
self._decoder = None
- self._pending = ""
- self._snapshot = None
+ self._decoded_text = "" # buffer for text produced by decoder
+ self._snapshot = None # info for reconstructing decoder state
self._seekable = self._telling = self.buffer.seekable()
+ # A word about _snapshot. This attribute is either None, or a tuple
+ # (decoder_state, input_chunk, decoded_chars) where decoder_state is
+ # the second (integer) item of the decoder state, input_chunk is the
+ # chunk of bytes that was read, and decoded_chars is the number of
+ # characters rendered by the decoder after feeding it those bytes.
+ # We use this to reconstruct intermediate decoder states in tell().
+
+ # Naming convention:
+ # - integer variables ending in "_bytes" count input bytes
+ # - integer variables ending in "_chars" count decoded characters
+
+ def __repr__(self):
+ return '<TIOW %x>' % id(self)
+
@property
def encoding(self):
return self._encoding
@@ -1196,13 +1209,6 @@
def line_buffering(self):
return self._line_buffering
- # A word about _snapshot. This attribute is either None, or a
- # tuple (decoder_state, readahead, pending) where decoder_state is
- # the second (integer) item of the decoder state, readahead is the
- # chunk of bytes that was read, and pending is the characters that
- # were rendered by the decoder after feeding it those bytes. We
- # use this to reconstruct intermediate decoder states in tell().
-
def seekable(self):
return self._seekable
@@ -1262,126 +1268,199 @@
return decoder
def _read_chunk(self):
+ """
+ Read and decode the next chunk of data from the BufferedReader.
+
+ Return a tuple of two elements: all the bytes that were read, and
+ the decoded string produced by the decoder. (The entire input
+ chunk is sent to the decoder, but some of it may remain buffered
+ in the decoder, yet to be converted.)
+ """
+
if self._decoder is None:
raise ValueError("no decoder")
if not self._telling:
- readahead = self.buffer.read1(self._CHUNK_SIZE)
- pending = self._decoder.decode(readahead, not readahead)
- return readahead, pending
- decoder_buffer, decoder_state = self._decoder.getstate()
- readahead = self.buffer.read1(self._CHUNK_SIZE)
- pending = self._decoder.decode(readahead, not readahead)
- self._snapshot = (decoder_state, decoder_buffer + readahead, pending)
- return readahead, pending
+ # No one should call tell(), so don't bother taking a snapshot.
+ input_chunk = self.buffer.read1(self._CHUNK_SIZE)
+ eof = not input_chunk
+ decoded = self._decoder.decode(input_chunk, eof)
+ return (input_chunk, decoded)
- def _encode_decoder_state(self, ds, pos):
- x = 0
- for i in bytes(ds):
- x = x<<8 | i
- return (x<<64) | pos
+ # The cookie returned by tell() cannot include the contents of
+ # the decoder's buffer, so we need to snapshot a point in the
+ # input where the decoder has nothing in its input buffer.
- def _decode_decoder_state(self, pos):
- x, pos = divmod(pos, 1<<64)
- if not x:
- return None, pos
- b = b""
- while x:
- b.append(x&0xff)
- x >>= 8
- return str(b[::-1]), pos
+ dec_buffer, dec_flags = self._decoder.getstate()
+ # The state tuple returned by getstate() contains the decoder's
+ # input buffer and an integer representing any other state. Thus,
+ # there is a valid snapshot point len(decoder_buffer) bytes ago in
+ # the input, with the state tuple (b'', decoder_state).
+
+ input_chunk = self.buffer.read1(self._CHUNK_SIZE)
+ eof = not input_chunk
+ decoded = self._decoder.decode(input_chunk, eof)
+
+ # At the snapshot point len(dec_buffer) bytes ago, the next input
+ # to be passed to the decoder is dec_buffer + input_chunk. Save
+ # len(decoded) so that later, tell() can figure out how much
+ # decoded data has been used up by TextIOWrapper.read().
+ self._snapshot = (dec_flags, dec_buffer + input_chunk, len(decoded))
+ return (input_chunk, decoded)
+
+ def _encode_tell_cookie(self, position, dec_flags=0,
+ feed_bytes=0, need_eof=0, skip_chars=0):
+ # The meaning of a tell() cookie is: seek to position, set the
+ # decoder flags to dec_flags, read feed_bytes bytes, feed them
+ # into the decoder with need_eof as the EOF flag, then skip
+ # skip_chars characters of the decoded result. For most simple
+ # decoders, this should often just be the position.
+ return (position | (dec_flags<<64) | (feed_bytes<<128) |
+ (skip_chars<<192) | bool(need_eof)<<256)
+
+ def _decode_tell_cookie(self, bigint):
+ rest, position = divmod(bigint, 1<<64)
+ rest, dec_flags = divmod(rest, 1<<64)
+ rest, feed_bytes = divmod(rest, 1<<64)
+ need_eof, skip_chars = divmod(rest, 1<<64)
+ return position, dec_flags, feed_bytes, need_eof, skip_chars
def tell(self):
if not self._seekable:
- raise IOError("Underlying stream is not seekable")
+ raise IOError("underlying stream is not seekable")
if not self._telling:
- raise IOError("Telling position disabled by next() call")
+ raise IOError("telling position disabled by next() call")
self.flush()
position = self.buffer.tell()
decoder = self._decoder
if decoder is None or self._snapshot is None:
- if self._pending:
- raise ValueError("pending data")
+ if self._decoded_text:
+ # This should never happen.
+ raise AssertionError("pending decoded text")
return position
- decoder_state, readahead, pending = self._snapshot
- position -= len(readahead)
- needed = len(pending) - len(self._pending)
- if not needed:
- return self._encode_decoder_state(decoder_state, position)
+
+ # Skip backward to the snapshot point (see _read_chunk).
+ dec_flags, next_input, decoded_chars = self._snapshot
+ position -= len(next_input)
+
+ # How many decoded characters have been consumed since the snapshot?
+ skip_chars = decoded_chars - len(self._decoded_text)
+ if skip_chars == 0:
+ # We haven't moved from the snapshot point.
+ return self._encode_tell_cookie(position, dec_flags)
+
+ # Walk the decoder forward, one byte at a time, to find the minimum
+ # input necessary to give us the decoded characters we need to skip.
+ # As we go, look for the "safe point" nearest to the current location
+ # (i.e. a point where the decoder has nothing buffered, so we can
+ # safely start from there when trying to return to this location).
saved_state = decoder.getstate()
try:
- decoder.setstate((b"", decoder_state))
- n = 0
- bb = bytearray(1)
- for i, bb[0] in enumerate(readahead):
- n += len(decoder.decode(bb))
- if n >= needed:
- decoder_buffer, decoder_state = decoder.getstate()
- return self._encode_decoder_state(
- decoder_state,
- position + (i+1) - len(decoder_buffer) - (n - needed))
- raise IOError("Can't reconstruct logical file position")
+ decoder.setstate((b"", dec_flags))
+ fed_bytes = 0
+ decoded_chars = 0
+ need_eof = 0
+ last_safe_point = (dec_flags, 0, 0)
+
+ next_byte = bytearray(1)
+ for next_byte[0] in next_input:
+ decoded = decoder.decode(next_byte)
+ fed_bytes += 1
+ decoded_chars += len(decoded)
+ dec_buffer, dec_flags = decoder.getstate()
+ if not dec_buffer and decoded_chars <= skip_chars:
+ # Decoder buffer is empty, so it's safe to start from here.
+ last_safe_point = (dec_flags, fed_bytes, decoded_chars)
+ if decoded_chars >= skip_chars:
+ break
+ else:
+ # We didn't get enough decoded data; send EOF to get more.
+ decoded = decoder.decode(b"", True)
+ decoded_chars += len(decoded)
+ need_eof = 1
+ if decoded_chars < skip_chars:
+ raise IOError("can't reconstruct logical file position")
+
+ # Advance the starting position to the last safe point.
+ dec_flags, safe_fed_bytes, safe_decoded_chars = last_safe_point
+ position += safe_fed_bytes
+ fed_bytes -= safe_fed_bytes
+ skip_chars -= safe_decoded_chars
+ return self._encode_tell_cookie(
+ position, dec_flags, fed_bytes, need_eof, skip_chars)
finally:
decoder.setstate(saved_state)
- def seek(self, pos, whence=0):
+ def seek(self, cookie, whence=0):
if not self._seekable:
- raise IOError("Underlying stream is not seekable")
- if whence == 1:
- if pos != 0:
- raise IOError("Can't do nonzero cur-relative seeks")
- pos = self.tell()
+ raise IOError("underlying stream is not seekable")
+ if whence == 1: # seek relative to current position
+ if cookie != 0:
+ raise IOError("can't do nonzero cur-relative seeks")
+ # Seeking to the current position should attempt to
+ # sync the underlying buffer with the current position.
whence = 0
- if whence == 2:
- if pos != 0:
- raise IOError("Can't do nonzero end-relative seeks")
+ cookie = self.tell()
+ if whence == 2: # seek relative to end of file
+ if cookie != 0:
+ raise IOError("can't do nonzero end-relative seeks")
self.flush()
- pos = self.buffer.seek(0, 2)
+ position = self.buffer.seek(0, 2)
+ self._decoded_text = ""
self._snapshot = None
- self._pending = ""
if self._decoder:
self._decoder.reset()
- return pos
+ return position
if whence != 0:
- raise ValueError("Invalid whence (%r, should be 0, 1 or 2)" %
+ raise ValueError("invalid whence (%r, should be 0, 1 or 2)" %
(whence,))
- if pos < 0:
- raise ValueError("Negative seek position %r" % (pos,))
+ if cookie < 0:
+ raise ValueError("negative seek position %r" % (cookie,))
self.flush()
- orig_pos = pos
- ds, pos = self._decode_decoder_state(pos)
- if not ds:
- self.buffer.seek(pos)
- self._snapshot = None
- self._pending = ""
- if self._decoder:
- self._decoder.reset()
- return pos
- decoder = self._decoder or self._get_decoder()
- decoder.set_state(("", ds))
- self.buffer.seek(pos)
- self._snapshot = (ds, b"", "")
- self._pending = ""
- self._decoder = decoder
- return orig_pos
+
+ # Seek back to the snapshot point.
+ position, dec_flags, feed_bytes, need_eof, skip_chars = \
+ self._decode_tell_cookie(cookie)
+ self.buffer.seek(position)
+ self._decoded_text = ""
+ self._snapshot = None
+
+ if self._decoder or dec_flags or feed_bytes or need_eof:
+ # Restore the decoder flags to their values from the snapshot.
+ self._decoder = self._decoder or self._get_decoder()
+ self._decoder.setstate((b"", dec_flags))
+
+ if feed_bytes or need_eof:
+ # Feed feed_bytes bytes to the decoder.
+ input_chunk = self.buffer.read(feed_bytes)
+ decoded = self._decoder.decode(input_chunk, need_eof)
+ if len(decoded) < skip_chars:
+ raise IOError("can't restore logical file position")
+
+ # Skip skip_chars of the decoded characters.
+ self._decoded_text = decoded[skip_chars:]
+
+ # Restore the snapshot.
+ self._snapshot = (dec_flags, input_chunk, len(decoded))
+ return cookie
def read(self, n=None):
if n is None:
n = -1
decoder = self._decoder or self._get_decoder()
- res = self._pending
+ result = self._decoded_text
if n < 0:
- res += decoder.decode(self.buffer.read(), True)
- self._pending = ""
+ result += decoder.decode(self.buffer.read(), True)
+ self._decoded_text = ""
self._snapshot = None
- return res
+ return result
else:
- while len(res) < n:
- readahead, pending = self._read_chunk()
- res += pending
- if not readahead:
+ while len(result) < n:
+ input_chunk, decoded = self._read_chunk()
+ result += decoded
+ if not input_chunk:
break
- self._pending = res[n:]
- return res[:n]
+ self._decoded_text = result[n:]
+ return result[:n]
def __next__(self):
self._telling = False
@@ -1400,10 +1479,11 @@
line = self.readline()
if len(line) <= limit:
return line
- line, self._pending = line[:limit], line[limit:] + self._pending
+ line, self._decoded_text = \
+ line[:limit], line[limit:] + self._decoded_text
return line
- line = self._pending
+ line = self._decoded_text
start = 0
decoder = self._decoder or self._get_decoder()
@@ -1467,11 +1547,11 @@
line += more_line
else:
# end of file
- self._pending = ''
+ self._decoded_text = ''
self._snapshot = None
return line
- self._pending = line[endpos:]
+ self._decoded_text = line[endpos:]
return line[:endpos]
@property