New submission from Eric Snow <ericsnowcurren...@gmail.com>:
(see http://mail.python.org/pipermail/python-dev/2012-April/118889.html)
The behavior of tokenize.detect_encoding() and
PyTokenizer_FindEncodingFilename() is unexpectedly different and this has
bearing on the current work on imports.
When a file has no encoding indicator (see PEP 263) it falls back to UTF8 (see
PEP 3120). The tokenize module (Lib/tokenize.py) facilitates this through
"detect_encoding()". The CPython internal tokenizer (Python/tokenizer.c) does
so through "PyTokenizer_FindEncodingFilename()". Both check the first two
lines of the file, per PEP 263.
When faced with an unparsable file (per the encoding),
tokenize.detect_encoding() will gladly give you the encoding without any fuss.
However, PyTokenizer_FindEncodingFilename() will raise a SyntaxError in that
situation.
The 'badsyntax_pep3120' test (Lib/test/badsyntax_pep3120.py) is one module that
demonstrates this discrepency. I'll use it in the following example.
---
For tokenize.detect_encoding():
import tokenize
enc =
tokenize.detect_encoding(open("cpython/Lib/test/badsyntax_pep3120.py").readline)
print(enc) # "utf-8" (no SyntaxError)
For PyTokenizer_FindEncodingFilename():
I've attached the source for a C extension module ('_tokenizer') that wraps
PyTokenizer_FindEncodingFilename().
import _tokenizer
enc = _tokenizer.detect_encoding("cpython/Lib/test/badsyntax_pep3120.py")
print(enc) # raises SyntaxError
---
Some relevant, related notes:
The discrepencies extend further too. The following code returns a
UnicodeDecodeError, rather than a SyntaxError:
tokenize.tokenize(open("/home/esnow/projects/import_cleanup/Lib/test/badsyntax_pep3120.py").readline)
In 3.1 (C-based import machinery, Python/import.c), the following results in a
SyntaxError, during encoding detection. In the current repo tip
(importlib-based import machinery, Lib/importlib/_bootstrap.py), the following
results in a SyntaxError much later, during compilation.
import test.badsyntax_pep3120
importlib uses tokenize.detect_encoding() and import.c uses
PyTokenizer_FindEncodingFilename()...
----------
components: Library (Lib)
files: _tokenizer.c
messages: 158797
nosy: brett.cannon, eric.snow, loewis
priority: normal
severity: normal
status: open
title: discrepency between tokenize.detect_encoding() and
PyTokenizer_FindEncodingFilename()
type: behavior
versions: Python 3.3
Added file: http://bugs.python.org/file25283/_tokenizer.c
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<http://bugs.python.org/issue14629>
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