在Python中使用/ import语句进行模拟修补

Question

I am trying to get mock.patch to work on the following piece of sample code:

from mock import patch
from collections import defaultdict

with patch('collections.defaultdict'):
  d = defaultdict()
  print 'd:', d

This outputs the following:

d: defaultdict(None, {})

Which means that defaultdict was not patched.

If I replace the from/import statement with a straight import statement it works:

from mock import patch
import collections

with patch('collections.defaultdict'):
 d = collections.defaultdict()
 print 'd:', d

Output is:

d: <MagicMock name='defaultdict()' id='139953944084176'>

Is there any way to patch a call using from/import?

Thank you

Answer 1

If you're patching something in the same module, you can use __main__ :

from mock import patch
from collections import defaultdict

with patch('__main__.defaultdict'):
    d = defaultdict()
    print 'd:', d

If you're mocking something for an imported module, however, you'll want to use that module's name so the correct reference (or name) is patched:

# foo.py

from collections import defaultdict

def bar():
    return defaultdict()


# foo_test.py    

from mock import patch
from foo import bar

with patch('foo.defaultdict'):
    print bar()

The point here is that patch wants the full path to the thing it is patching. This just looks a little weird when patching something in the current module, since folks don't often use __main__ (or have to refer to the current module, for that matter).

Answer 2

patch works by patching names . You can't achieve anything by patching the name collections.defaultdict if you are using the name defaultdict (in the local namespace) to access the object. See the documentation at http://www.voidspace.org.uk/python/mock/patch.html#id1 .

Answer 3

The names can be very confusing in this case. We always want to mock a class definition in a namespace. The namespace is the module in which the import takes place. The class definition is the name used in that namespace.

Let's take a concrete example:

• myproj.utilities module contains Actor class
• myproj.application imports this as from myproj.utilities import Actor
• my test should execute my.proj.application and mock our Actor

myproj.utilities.py

class Actor:
    def __init__(name):
        self.name = name

myproj.application.py

from myproj.utilities import Actor

class App:
    def __init__(name):
        self.actor = Actor(name)

test code

from mock import patch
from myproj.application import App

test:
  # format: patch('<namespace>.<Class>')
  # the namespace in which we with to mock
  # the class definition we wish to mock
  with patch('myproj.application.Actor'):
      app = App('Someone')
      print( type(app.actor) ) # expect a MagicMock

I tried several other approaches and this one works well for me. I have not tests the code above but instead generalized it from my own specific case. So, it might be a little off.