[Update: This is a known bug.]
Is there a way to use read(some_number_of_bytes)
with mock_open
?
mock_open
works as expected when reading the entire file (read()
) or when reading a single line (readline()
), but I can't get it working when reading a number of bytes (read(n)
)
These snippets (adapted from the Mock documentation) work (Python 2.7):
from mock import mock_open, patch
# works: consume entire "file"
with patch('__main__.open', mock_open(read_data='bibble')) as m:
with open('foo') as h:
result = h.read()
assert result == 'bibble' # ok
# works: consume one line
with patch('__main__.open', mock_open(read_data='bibble\nbobble')) as m:
with open('foo') as h:
result = h.readline()
assert result == 'bibble\n' # ok
But trying to read only a few bytes fails--mock_open
returns the entire read_data
instead:
# consume first 3 bytes of the "file"
with patch('__main__.open', mock_open(read_data='bibble')) as m:
with open('foo') as h:
result = h.read(3)
assert result == 'bib', 'result of read: {}'.format(result) # fails
Output:
Traceback (most recent call last):
File "/tmp/t.py", line 25, in <module>
assert result == 'bib', 'result of read: {}'.format(result)
AssertionError: result of read: bibble
In case this is an XY problem, I'll note that the reason for this question is that I'm mocking a file that I pass into pickle.load
, which in turn calls read(1)
.
with open('/path/to/file.pkl', 'rb') as f:
x = pickle.load(f) # this requires f.read(1) to work
I'd prefer to mock the lowest level possible (open
rather than pickle.load
).
For the record, this issue has been fixed in Python 3.7.