[issue24159] Misleading TypeError when pickling bytes to a file opened as text

[stein-k]

Change by stein-k :


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nosy: +stein-k

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[issue24159] Misleading TypeError when pickling bytes to a file opened as text

[Борис Верховский]

Борис Верховский  added the comment:

As I said in issue38226, the error message you get when you try to 
pickle.load() a file opened in "r" mode instead of "rb" mode,

UnicodeDecodeError: 'utf-8' codec can't decode byte 0x80 in position 0: invalid 
start byte

is also confusing and can be improved.

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[issue24159] Misleading TypeError when pickling bytes to a file opened as text

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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versions: +Python 3.8 -Python 3.7

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[issue24159] Misleading TypeError when pickling bytes to a file opened as text

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

I think it is worth to improve the error message in the write() method of 
binary files.

>>> sys.stdout.write(b'')
Traceback (most recent call last):
  File "", line 1, in 
TypeError: write() argument must be str, not bytes
>>> sys.stdout.buffer.write('')
Traceback (most recent call last):
  File "", line 1, in 
TypeError: a bytes-like object is required, not 'str'
>>> sys.stdout.buffer.raw.write('')
Traceback (most recent call last):
  File "", line 1, in 
TypeError: a bytes-like object is required, not 'str'

But this is large issue. Other file-like objects (GzipFile, ZipExtFile etc) 
should be updated too.

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nosy: +serhiy.storchaka
type: behavior -> enhancement
versions: +Python 3.7 -Python 3.5

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[issue24159] Misleading TypeError when pickling bytes to a file opened as text

[Ned Deily]

Changes by Ned Deily n...@acm.org:


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nosy: +alexandre.vassalotti, pitrou

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[issue24159] Misleading TypeError when pickling bytes to a file opened as text

[Jon Clements]

Changes by Jon Clements jon...@googlemail.com:


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nosy: +joncle

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[issue24159] Misleading TypeError when pickling bytes to a file opened as text

[Jason R. Coombs]

New submission from Jason R. Coombs:

I had a piece of code which I distilled to this:

import pickle
with open('out.pickle', 'w') as out:
pickle.dump(out, b'data')

Running that code raises this error:

TypeError: must be str, not bytes

The error is raised at the dump call with no additional context. Based on the 
error, my reaction is to think that pickled doesn't support bytes objects in 
pickles.

On further examination, it's not actually that the bytes cannot be pickled, but 
that the 'dump' call requires that the file be opened in binary mode ('wb'), 
but because of the error message essentially says expecting a text string and 
because it's unclear that the error is raised during the write to the stream 
and because the JSON library expects an output stream to be opened in text 
mode, the error message is misleading.

At least [two other people think the behavior could be 
clearer](http://stackoverflow.com/questions/13906623/using-pickle-dump-typeerror-must-be-str-not-bytes#comment43761528_13906715).

Would it be possible and reasonable to trap a TypeError at the call to `.write` 
and replace or augment the message with something like file must be opened in 
binary mode?

On second thought, perhaps the culprit isn't pickle here, but the stream 
writer. Perhaps the `.write` method should provide a clearer message indicating 
the context at which it's expecting str and not bytes.

--
components: IO
messages: 242858
nosy: jason.coombs
priority: low
severity: normal
status: open
title: Misleading TypeError when pickling bytes to a file opened as text
type: behavior
versions: Python 3.5

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