Python – How to stub Python methods without Mock

mockingpythonstubunit testing

I'm a C# dev moving into some Python stuff, so I don't know what I'm doing just yet. I've read that you don't really need Dependency Injection with Python. I've been told you can instantiate objects in your code and have them run the way you want, however, you can point methods on those objects to my own stubs defined in my tests – supposedly without mocks.

Is this true? I've tried to do it and can't quite get it working. How is this actually done? How do I stub a method in Python without a mocking library?

Best Answer

Here's a basic example. Note that the production getData() method is never called. It has been mocked out with a stub.

import unittest
class ClassIWantToTest(object):

    def getData(self):
        print "PRODUCTION getData called"
        return "Production code that gets data from server or data file"

    def getDataLength(self):
        return len(self.getData())

class TestClassIWantToTest(unittest.TestCase):

    def testGetDataLength(self):
        def mockGetData(self):
            print "MOCK getData called"
            return "1234"

        origGetData = ClassIWantToTest.getData
        try:
            ClassIWantToTest.getData = mockGetData
            myObj = ClassIWantToTest()
            self.assertEqual(4, myObj.getDataLength())
        finally:
            ClassIWantToTest.getData = origGetData

if __name__ == "__main__":
    unittest.main()

Related Solutions

Java – How to test a class that has private methods, fields or inner classes

Update:

Some 10 years later perhaps the best way to test a private method, or any inaccessible member, is via @Jailbreak from the Manifold framework.
@Jailbreak Foo foo = new Foo();
// Direct, *type-safe* access to *all* foo's members
foo.privateMethod(x, y, z);
foo.privateField = value;
This way your code remains type-safe and readable. No design compromises, no overexposing methods and fields for the sake of tests.

If you have somewhat of a legacy Java application, and you're not allowed to change the visibility of your methods, the best way to test private methods is to use reflection.

Internally we're using helpers to get/set private and private static variables as well as invoke private and private static methods. The following patterns will let you do pretty much anything related to the private methods and fields. Of course, you can't change private static final variables through reflection.

Method method = TargetClass.getDeclaredMethod(methodName, argClasses);
method.setAccessible(true);
return method.invoke(targetObject, argObjects);

And for fields:

Field field = TargetClass.getDeclaredField(fieldName);
field.setAccessible(true);
field.set(object, value);

Notes:
1. TargetClass.getDeclaredMethod(methodName, argClasses) lets you look into private methods. The same thing applies for getDeclaredField.
2. The setAccessible(true) is required to play around with privates.

Python – How to check whether a file exists without exceptions

If the reason you're checking is so you can do something like if file_exists: open_it(), it's safer to use a try around the attempt to open it. Checking and then opening risks the file being deleted or moved or something between when you check and when you try to open it.

If you're not planning to open the file immediately, you can use os.path.isfile

Return True if path is an existing regular file. This follows symbolic links, so both islink() and isfile() can be true for the same path.

import os.path
os.path.isfile(fname)

if you need to be sure it's a file.

Starting with Python 3.4, the pathlib module offers an object-oriented approach (backported to pathlib2 in Python 2.7):

from pathlib import Path

my_file = Path("/path/to/file")
if my_file.is_file():
    # file exists

To check a directory, do:

if my_file.is_dir():
    # directory exists

To check whether a Path object exists independently of whether is it a file or directory, use exists():

if my_file.exists():
    # path exists

You can also use resolve(strict=True) in a try block:

try:
    my_abs_path = my_file.resolve(strict=True)
except FileNotFoundError:
    # doesn't exist
else:
    # exists

Best Answer

Related Solutions

Java – How to test a class that has private methods, fields or inner classes

Python – How to check whether a file exists without exceptions

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