[issue27197] mock.patch interactions with "from" imports

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan :


--
nosy: +xtreak

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[issue27197] mock.patch interactions with "from" imports

[Robert Collins]

Robert Collins added the comment:

So its a feature of mock that it can mock a module that doesn't exist. And the 
semantics of the import system are designed to be very cheap when a module is 
already imported - so when 'patchbug.a' is in sys.modules, import will 
correctly return it rather than going out to the import path to find if its the 
same thing.

We could possibly have an opt-in check on mock.patch to require that the thing 
being patched exists, but it would have to be opt-in as so many cases are 
dealing with optional or pretend instances. I'm not super keen on that though.

This is perhaps worth calling out in the docs to avoid confusion.

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nosy: +rbcollins

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[issue27197] mock.patch interactions with "from" imports

[Clark Breyman]

New submission from Clark Breyman:

Unclear if this is a code bug or a gotcha that should be documented:
Cross posted to https://github.com/testing-cabal/mock/issues/365

Since "from"-style imports in modules bind names on the first execution of the 
import, there is a subtle interaction with patching:

_If a name is imported when the source is patched, the imported name will refer 
to the patched version even after the source is restored_

Test case:
```python
# patchbug/__init__.py
#
```

```python
# patchbug/a.py
#
class A(object):
def name(self):
return "unpatched"
```

```python
# patchbug/b.py
#
from patchbug.a import A

def reference():
return A().name()

# patchbug/tests.py
#
import mock

"""
patchbug.reference.UnpatchedClass is bound to the value of 
patchbug.source.UnpatchedClass
at the time of the first import of patchbug.reference. If patched at that time 
it is not repaired.
"""

def test_unpatched():
import patchbug.a
assert patchbug.a.A().name() == "unpatched"

"""
uncommenting the import causes reference.UnpatchedClass to be bound before the 
patch, fixing
test_reference and breaking test_unpatched
"""
def test_import_reference():
#import patchbug.b
pass

def test_patched():
with mock.patch('patchbug.a.A') as P:
import patchbug.a, patchbug.b
P.return_value.name.return_value = "patched"
assert patchbug.a.A().name() == "patched"
assert patchbug.b.reference() == "patched"

def test_reference():
import patchbug.b
assert patchbug.b.reference() == "unpatched"
```

--
components: Library (Lib)
messages: 267114
nosy: clarkbreyman
priority: normal
severity: normal
status: open
title: mock.patch interactions with "from" imports
type: behavior
versions: Python 3.5

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