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.
This commit is contained in:
Ka-Ping Yee
280
Lib/io.py
280
Lib/io.py
@@ -802,11 +802,10 @@ class BufferedReader(_BufferedIOMixin):
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return self._read_buf
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def read1(self, n):
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"""Reads up to n bytes.
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"""Reads up to n bytes, with at most one read() system call.
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Returns up to n bytes. If at least one byte is buffered,
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we only return buffered bytes. Otherwise, we do one
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raw read.
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Returns up to n bytes. If at least one byte is buffered, we
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only return buffered bytes. Otherwise, we do one raw read.
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"""
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if n <= 0:
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return b""
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@@ -1180,10 +1179,24 @@ class TextIOWrapper(TextIOBase):
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self._writenl = newline or os.linesep
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self._encoder = None
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self._decoder = None
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self._pending = ""
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self._snapshot = None
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self._decoded_text = "" # buffer for text produced by decoder
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self._snapshot = None # info for reconstructing decoder state
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self._seekable = self._telling = self.buffer.seekable()
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# A word about _snapshot. This attribute is either None, or a tuple
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# (decoder_state, input_chunk, decoded_chars) where decoder_state is
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# the second (integer) item of the decoder state, input_chunk is the
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# chunk of bytes that was read, and decoded_chars is the number of
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# characters rendered by the decoder after feeding it those bytes.
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# We use this to reconstruct intermediate decoder states in tell().
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# Naming convention:
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# - integer variables ending in "_bytes" count input bytes
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# - integer variables ending in "_chars" count decoded characters
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def __repr__(self):
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return '<TIOW %x>' % id(self)
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@property
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def encoding(self):
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return self._encoding
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@@ -1196,13 +1209,6 @@ class TextIOWrapper(TextIOBase):
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def line_buffering(self):
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return self._line_buffering
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# A word about _snapshot. This attribute is either None, or a
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# tuple (decoder_state, readahead, pending) where decoder_state is
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# the second (integer) item of the decoder state, readahead is the
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# chunk of bytes that was read, and pending is the characters that
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# were rendered by the decoder after feeding it those bytes. We
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# use this to reconstruct intermediate decoder states in tell().
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def seekable(self):
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return self._seekable
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@@ -1262,126 +1268,199 @@ class TextIOWrapper(TextIOBase):
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return decoder
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def _read_chunk(self):
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"""
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Read and decode the next chunk of data from the BufferedReader.
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Return a tuple of two elements: all the bytes that were read, and
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the decoded string produced by the decoder. (The entire input
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chunk is sent to the decoder, but some of it may remain buffered
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in the decoder, yet to be converted.)
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"""
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if self._decoder is None:
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raise ValueError("no decoder")
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if not self._telling:
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readahead = self.buffer.read1(self._CHUNK_SIZE)
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pending = self._decoder.decode(readahead, not readahead)
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return readahead, pending
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decoder_buffer, decoder_state = self._decoder.getstate()
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readahead = self.buffer.read1(self._CHUNK_SIZE)
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pending = self._decoder.decode(readahead, not readahead)
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self._snapshot = (decoder_state, decoder_buffer + readahead, pending)
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return readahead, pending
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# No one should call tell(), so don't bother taking a snapshot.
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input_chunk = self.buffer.read1(self._CHUNK_SIZE)
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eof = not input_chunk
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decoded = self._decoder.decode(input_chunk, eof)
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return (input_chunk, decoded)
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def _encode_decoder_state(self, ds, pos):
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x = 0
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for i in bytes(ds):
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x = x<<8 | i
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return (x<<64) | pos
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# The cookie returned by tell() cannot include the contents of
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# the decoder's buffer, so we need to snapshot a point in the
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# input where the decoder has nothing in its input buffer.
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def _decode_decoder_state(self, pos):
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x, pos = divmod(pos, 1<<64)
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if not x:
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return None, pos
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b = b""
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while x:
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b.append(x&0xff)
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x >>= 8
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return str(b[::-1]), pos
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dec_buffer, dec_flags = self._decoder.getstate()
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# The state tuple returned by getstate() contains the decoder's
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# input buffer and an integer representing any other state. Thus,
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# there is a valid snapshot point len(decoder_buffer) bytes ago in
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# the input, with the state tuple (b'', decoder_state).
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input_chunk = self.buffer.read1(self._CHUNK_SIZE)
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eof = not input_chunk
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decoded = self._decoder.decode(input_chunk, eof)
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# At the snapshot point len(dec_buffer) bytes ago, the next input
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# to be passed to the decoder is dec_buffer + input_chunk. Save
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# len(decoded) so that later, tell() can figure out how much
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# decoded data has been used up by TextIOWrapper.read().
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self._snapshot = (dec_flags, dec_buffer + input_chunk, len(decoded))
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return (input_chunk, decoded)
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def _encode_tell_cookie(self, position, dec_flags=0,
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feed_bytes=0, need_eof=0, skip_chars=0):
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# The meaning of a tell() cookie is: seek to position, set the
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# decoder flags to dec_flags, read feed_bytes bytes, feed them
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# into the decoder with need_eof as the EOF flag, then skip
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# skip_chars characters of the decoded result. For most simple
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# decoders, this should often just be the position.
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return (position | (dec_flags<<64) | (feed_bytes<<128) |
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(skip_chars<<192) | bool(need_eof)<<256)
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def _decode_tell_cookie(self, bigint):
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rest, position = divmod(bigint, 1<<64)
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rest, dec_flags = divmod(rest, 1<<64)
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rest, feed_bytes = divmod(rest, 1<<64)
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need_eof, skip_chars = divmod(rest, 1<<64)
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return position, dec_flags, feed_bytes, need_eof, skip_chars
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def tell(self):
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if not self._seekable:
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raise IOError("Underlying stream is not seekable")
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raise IOError("underlying stream is not seekable")
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if not self._telling:
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raise IOError("Telling position disabled by next() call")
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raise IOError("telling position disabled by next() call")
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self.flush()
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position = self.buffer.tell()
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decoder = self._decoder
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if decoder is None or self._snapshot is None:
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if self._pending:
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raise ValueError("pending data")
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if self._decoded_text:
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# This should never happen.
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raise AssertionError("pending decoded text")
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return position
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decoder_state, readahead, pending = self._snapshot
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position -= len(readahead)
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needed = len(pending) - len(self._pending)
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if not needed:
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return self._encode_decoder_state(decoder_state, position)
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# Skip backward to the snapshot point (see _read_chunk).
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dec_flags, next_input, decoded_chars = self._snapshot
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position -= len(next_input)
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# How many decoded characters have been consumed since the snapshot?
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skip_chars = decoded_chars - len(self._decoded_text)
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if skip_chars == 0:
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# We haven't moved from the snapshot point.
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return self._encode_tell_cookie(position, dec_flags)
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# Walk the decoder forward, one byte at a time, to find the minimum
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# input necessary to give us the decoded characters we need to skip.
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# As we go, look for the "safe point" nearest to the current location
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# (i.e. a point where the decoder has nothing buffered, so we can
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# safely start from there when trying to return to this location).
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saved_state = decoder.getstate()
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try:
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decoder.setstate((b"", decoder_state))
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n = 0
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bb = bytearray(1)
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for i, bb[0] in enumerate(readahead):
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n += len(decoder.decode(bb))
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if n >= needed:
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decoder_buffer, decoder_state = decoder.getstate()
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return self._encode_decoder_state(
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decoder_state,
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position + (i+1) - len(decoder_buffer) - (n - needed))
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raise IOError("Can't reconstruct logical file position")
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decoder.setstate((b"", dec_flags))
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fed_bytes = 0
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decoded_chars = 0
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need_eof = 0
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last_safe_point = (dec_flags, 0, 0)
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next_byte = bytearray(1)
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for next_byte[0] in next_input:
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decoded = decoder.decode(next_byte)
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fed_bytes += 1
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decoded_chars += len(decoded)
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dec_buffer, dec_flags = decoder.getstate()
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if not dec_buffer and decoded_chars <= skip_chars:
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# Decoder buffer is empty, so it's safe to start from here.
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last_safe_point = (dec_flags, fed_bytes, decoded_chars)
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if decoded_chars >= skip_chars:
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break
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else:
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# We didn't get enough decoded data; send EOF to get more.
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decoded = decoder.decode(b"", True)
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decoded_chars += len(decoded)
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need_eof = 1
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if decoded_chars < skip_chars:
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raise IOError("can't reconstruct logical file position")
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# Advance the starting position to the last safe point.
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dec_flags, safe_fed_bytes, safe_decoded_chars = last_safe_point
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position += safe_fed_bytes
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fed_bytes -= safe_fed_bytes
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skip_chars -= safe_decoded_chars
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return self._encode_tell_cookie(
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position, dec_flags, fed_bytes, need_eof, skip_chars)
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finally:
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decoder.setstate(saved_state)
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def seek(self, pos, whence=0):
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def seek(self, cookie, whence=0):
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if not self._seekable:
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raise IOError("Underlying stream is not seekable")
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if whence == 1:
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if pos != 0:
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raise IOError("Can't do nonzero cur-relative seeks")
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pos = self.tell()
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raise IOError("underlying stream is not seekable")
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if whence == 1: # seek relative to current position
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if cookie != 0:
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raise IOError("can't do nonzero cur-relative seeks")
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# Seeking to the current position should attempt to
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# sync the underlying buffer with the current position.
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whence = 0
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if whence == 2:
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if pos != 0:
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raise IOError("Can't do nonzero end-relative seeks")
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cookie = self.tell()
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if whence == 2: # seek relative to end of file
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if cookie != 0:
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raise IOError("can't do nonzero end-relative seeks")
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self.flush()
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pos = self.buffer.seek(0, 2)
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position = self.buffer.seek(0, 2)
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self._decoded_text = ""
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self._snapshot = None
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self._pending = ""
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if self._decoder:
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self._decoder.reset()
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return pos
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return position
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if whence != 0:
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raise ValueError("Invalid whence (%r, should be 0, 1 or 2)" %
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raise ValueError("invalid whence (%r, should be 0, 1 or 2)" %
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(whence,))
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if pos < 0:
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raise ValueError("Negative seek position %r" % (pos,))
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if cookie < 0:
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raise ValueError("negative seek position %r" % (cookie,))
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self.flush()
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orig_pos = pos
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ds, pos = self._decode_decoder_state(pos)
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if not ds:
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self.buffer.seek(pos)
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self._snapshot = None
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self._pending = ""
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if self._decoder:
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self._decoder.reset()
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return pos
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decoder = self._decoder or self._get_decoder()
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decoder.set_state(("", ds))
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self.buffer.seek(pos)
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self._snapshot = (ds, b"", "")
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self._pending = ""
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self._decoder = decoder
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return orig_pos
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# Seek back to the snapshot point.
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position, dec_flags, feed_bytes, need_eof, skip_chars = \
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self._decode_tell_cookie(cookie)
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self.buffer.seek(position)
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self._decoded_text = ""
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self._snapshot = None
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if self._decoder or dec_flags or feed_bytes or need_eof:
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# Restore the decoder flags to their values from the snapshot.
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self._decoder = self._decoder or self._get_decoder()
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self._decoder.setstate((b"", dec_flags))
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if feed_bytes or need_eof:
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# Feed feed_bytes bytes to the decoder.
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input_chunk = self.buffer.read(feed_bytes)
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decoded = self._decoder.decode(input_chunk, need_eof)
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if len(decoded) < skip_chars:
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raise IOError("can't restore logical file position")
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# Skip skip_chars of the decoded characters.
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self._decoded_text = decoded[skip_chars:]
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# Restore the snapshot.
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self._snapshot = (dec_flags, input_chunk, len(decoded))
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return cookie
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def read(self, n=None):
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if n is None:
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n = -1
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decoder = self._decoder or self._get_decoder()
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res = self._pending
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result = self._decoded_text
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if n < 0:
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res += decoder.decode(self.buffer.read(), True)
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self._pending = ""
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result += decoder.decode(self.buffer.read(), True)
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self._decoded_text = ""
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self._snapshot = None
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return res
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return result
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else:
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while len(res) < n:
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readahead, pending = self._read_chunk()
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res += pending
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if not readahead:
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while len(result) < n:
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input_chunk, decoded = self._read_chunk()
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result += decoded
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if not input_chunk:
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break
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self._pending = res[n:]
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return res[:n]
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self._decoded_text = result[n:]
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return result[:n]
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def __next__(self):
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self._telling = False
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@@ -1400,10 +1479,11 @@ class TextIOWrapper(TextIOBase):
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line = self.readline()
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if len(line) <= limit:
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return line
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line, self._pending = line[:limit], line[limit:] + self._pending
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line, self._decoded_text = \
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line[:limit], line[limit:] + self._decoded_text
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return line
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line = self._pending
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line = self._decoded_text
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start = 0
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decoder = self._decoder or self._get_decoder()
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@@ -1467,11 +1547,11 @@ class TextIOWrapper(TextIOBase):
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line += more_line
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else:
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# end of file
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self._pending = ''
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self._decoded_text = ''
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self._snapshot = None
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return line
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self._pending = line[endpos:]
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self._decoded_text = line[endpos:]
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return line[:endpos]
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@property
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