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Title: mock_open() should allow reading binary data
Type: behavior Stage: resolved
Components: Library (Lib) Versions: Python 3.6, Python 3.4, Python 3.5
Status: closed Resolution: fixed
Dependencies: 17467 Superseder:
Assigned To: berker.peksag Nosy List: Aaron1011, a.badger, berker.peksag, jcea, michael.foord, python-dev, r.david.murray, rbcollins
Priority: normal Keywords: easy, patch

Created on 2014-12-07 15:24 by jcea, last changed 2022-04-11 14:58 by admin. This issue is now closed.

File name Uploaded Description Edit
mock-open-allow-binary-data.patch Aaron1011, 2014-12-13 03:07 review
mock-open-allow-binary-data-updated.patch Aaron1011, 2014-12-13 14:13 review
mock-open-allow-binary-without-coerce.patch Aaron1011, 2014-12-14 13:45 review
mock-open-allow-binary-without-coerce-fixup.patch Aaron1011, 2014-12-16 01:06 review
mock-open-allow-binary-data-fix-formatting.patch Aaron1011, 2014-12-20 02:04 review
Messages (18)
msg232271 - (view) Author: Jesús Cea Avión (jcea) * (Python committer) Date: 2014-12-07 15:24
mock_open(read_data=b'...') gives an error:

"""Traceback (most recent call last):
  File "", line 6, in <module>
  File "/usr/local/lib/python3.4/unittest/", line 896, in __call__
    return _mock_self._mock_call(*args, **kwargs)
  File "/usr/local/lib/python3.4/unittest/", line 962, in _mock_call
    ret_val = effect(*args, **kwargs)
  File "/usr/local/lib/python3.4/unittest/", line 2279, in _read_side_effect
    return ''.join(_data)
  File "/usr/local/lib/python3.4/unittest/", line 2244, in _iterate_read_data
    data_as_list = ['{}\n'.format(l) for l in read_data.split('\n')]

Easy to reproduce:

from unittest.mock import mock_open, patch

m = mock_open(read_data= b'abc')
with patch('', m, create=True) :
    with open('abc', 'rb') as f :

Looks like this bug was introduced as result of issue #17467. I add those people to the nosy list.
msg232589 - (view) Author: Aaron Hill (Aaron1011) * Date: 2014-12-13 03:07
I've created a patch that fixes this, and added an accompanying unit test (which fails without the change).
msg232591 - (view) Author: Demian Brecht (demian.brecht) * (Python triager) Date: 2014-12-13 03:41
Thanks for the patch Aaron. Unfortunately this doesn't quite fix the issue. There are two problems with the patch:

If a bytes object is passed into mock_open, I'd expect a bytes object in the output. In your patch, not only is this not the case (the output is a string object), but the bytes object is being coerced into its string representation in the resulting list. For example (simplified from your patch):

>>> data = b'foo\nbar'
>>> newline = b'\n'
>>> ['{}\n'.format(l) for l in data.split(newline)]
["b'foo'\n", "b'bar'\n"]

What I would expect to see in this case is:

[b'foo\n', b'bar\n']
msg232592 - (view) Author: Demian Brecht (demian.brecht) * (Python triager) Date: 2014-12-13 03:42
> There are two problems with the patch

That was intended to be removed after I changed the wording :P
msg232611 - (view) Author: Aaron Hill (Aaron1011) * Date: 2014-12-13 14:13
I've created a new patch, which addresses the problem. Your example now currently returns [b'foo\n', b'bar\n']
msg232629 - (view) Author: Demian Brecht (demian.brecht) * (Python triager) Date: 2014-12-14 04:54
Thanks for the update, but this doesn't quite work either as you're assuming utf-8 (which is what .encode() and .decode() default to). For example, when using latin-1:

>>> m = mock_open(read_data= b'\xc6')
>>> with patch('', m, create=True) :
...     with open('abc', 'rb') as f :
...         print(
Traceback (most recent call last):
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xc6 in position 0: unexpected end of data

Additionally, a bytes object may simply be binary data that doesn't adhere to any specific encoding.

My suggestion is to remove the use of format() altogether as it's really not doing anything complex and simply append either '\n' or b'\n' depending on the type of object passed in. That way, you can deal with the type of object passed in directly without coercion.
msg232637 - (view) Author: Aaron Hill (Aaron1011) * Date: 2014-12-14 13:45
Thanks, I've fixed that. Not sure why I thought decoding and re-encoding would work with any binary data.

I've also updated one of the tests to use non-utf8-decodeable binary data, to prevent a future regression.
msg232645 - (view) Author: Demian Brecht (demian.brecht) * (Python triager) Date: 2014-12-15 01:04
Thanks again for the update Aaron, I've left a couple small comments in Rietveld. Other than those, the patch looks good to me. Thanks for the contribution!
msg232693 - (view) Author: Aaron Hill (Aaron1011) * Date: 2014-12-16 01:06
I've fixed the issues you pointed out.

Is there a better way than uploading a new patch file to make changes?
msg232704 - (view) Author: Demian Brecht (demian.brecht) * (Python triager) Date: 2014-12-16 03:10
Thanks for the updated patch, looks good to me. If you haven't already read it, the patch workflow is here: and is the only workflow currently available.
msg232954 - (view) Author: Aaron Hill (Aaron1011) * Date: 2014-12-20 02:04
I've fixed the formatting issues.
msg233231 - (view) Author: Demian Brecht (demian.brecht) * (Python triager) Date: 2014-12-31 09:23
A few more comments were left in Rietveld for you, likely hidden by spam filters.
msg237020 - (view) Author: Berker Peksag (berker.peksag) * (Python committer) Date: 2015-03-02 05:46
msg248119 - (view) Author: Roundup Robot (python-dev) (Python triager) Date: 2015-08-06 10:17
New changeset 3d7adf5b3fb3 by Berker Peksag in branch '3.4':
Issue #23004: mock_open() now reads binary data correctly when the type of read_data is bytes.

New changeset 526a186de32d by Berker Peksag in branch '3.5':
Issue #23004: mock_open() now reads binary data correctly when the type of read_data is bytes.

New changeset ed15f399a292 by Berker Peksag in branch 'default':
Issue #23004: mock_open() now reads binary data correctly when the type of read_data is bytes.
msg248120 - (view) Author: Berker Peksag (berker.peksag) * (Python committer) Date: 2015-08-06 10:21
Thanks for the patch, Aaron(also thanks to Demian for reviews). I've fixed the merge conflict and added more tests.
msg248153 - (view) Author: Robert Collins (rbcollins) * (Python committer) Date: 2015-08-06 21:57
Post merge review:

looks like

data_as_list = read_data.splitlines(True)

would be a little cleaner.
msg248197 - (view) Author: Berker Peksag (berker.peksag) * (Python committer) Date: 2015-08-07 14:52
data_as_list = read_data.splitlines(True)

is not actually the equivalent of

    data_as_list = [l + sep for l in read_data.split(sep)]

It will change the behavior of the _iterate_read_data helper. See the comment at

However, in default branch, we can simplify it to

    yield from read_data.splitlines(keepends=True)
msg248201 - (view) Author: R. David Murray (r.david.murray) * (Python committer) Date: 2015-08-07 15:50
splitlines(keepends=True) is not ever equivalent to splitting by just '\n'.  I don't know the details here, but switching to that would certainly be a behavior change.  (Especially if the code path also applies to non-binary data!):

>>> b'abc\nde\r\nf\x1dg'.splitlines(True)
[b'abc\n', b'de\r\n', b'f\x1dg']
>>> 'abc\nde\r\nf\x1dg'.splitlines(True)
['abc\n', 'de\r\n', 'f\x1d', 'g']
Date User Action Args
2022-04-11 14:58:10adminsetgithub: 67193
2015-08-07 15:50:42r.david.murraysetnosy: + r.david.murray
messages: + msg248201
2015-08-07 14:52:39berker.peksagsetmessages: + msg248197
2015-08-06 21:57:48rbcollinssetmessages: + msg248153
2015-08-06 10:21:50berker.peksagsetstatus: open -> closed
resolution: fixed
messages: + msg248120

stage: commit review -> resolved
2015-08-06 10:17:42python-devsetnosy: + python-dev
messages: + msg248119
2015-08-06 09:18:50berker.peksagsetassignee: berker.peksag
2015-07-15 01:14:07rbcollinssetnosy: + rbcollins

versions: + Python 3.6
2015-03-02 05:46:01berker.peksagsetmessages: + msg237020
components: + Library (Lib)
stage: patch review -> commit review
2015-02-13 01:26:16demian.brechtsetnosy: - demian.brecht
2014-12-31 09:23:41demian.brechtsetmessages: + msg233231
2014-12-20 02:04:43Aaron1011setfiles: + mock-open-allow-binary-data-fix-formatting.patch

messages: + msg232954
2014-12-16 03:26:34berker.peksagsetstage: needs patch -> patch review
2014-12-16 03:10:32demian.brechtsetmessages: + msg232704
2014-12-16 01:06:12Aaron1011setfiles: + mock-open-allow-binary-without-coerce-fixup.patch

messages: + msg232693
2014-12-15 01:04:58demian.brechtsetmessages: + msg232645
2014-12-14 13:45:04Aaron1011setfiles: + mock-open-allow-binary-without-coerce.patch

messages: + msg232637
2014-12-14 04:54:56demian.brechtsetmessages: + msg232629
2014-12-13 14:13:42Aaron1011setfiles: + mock-open-allow-binary-data-updated.patch

messages: + msg232611
2014-12-13 03:42:30demian.brechtsetmessages: + msg232592
2014-12-13 03:41:18demian.brechtsetnosy: + demian.brecht
messages: + msg232591
2014-12-13 03:07:50Aaron1011setfiles: + mock-open-allow-binary-data.patch

nosy: + Aaron1011
messages: + msg232589

keywords: + patch
2014-12-07 16:09:22berker.peksagsetnosy: + berker.peksag
stage: needs patch
type: behavior

versions: + Python 3.4
2014-12-07 15:25:59jceasetdependencies: + Enhancement: give mock_open readline() and readlines() methods
2014-12-07 15:25:42jcealinkissue17467 superseder
2014-12-07 15:24:17jceacreate