python - how can I dynamically create a function with a name based on a string read in from a file?

Question

Let's say I have a file containing the string "unpredictable_words". I would like to read in this string and then define a function as follows:

def test_unpredictable_words(self):
    do_important_stuff()

I would then like to inject this function into a class definition so that this function can be called on any instances of this class.

How can I accomplish this?

I looked a bit at this answer - https://stackoverflow.com/a/8160676/1701170 - but I don't think it does what I want, exactly, or at least I can't understand what is going on.

warvariuc · Accepted Answer · 2012-12-29 03:08:04Z

5

Python 2.7.3 (default, Sep 26 2012, 21:51:14) 
>>> def injected(self):
...     print 'injected'
... 
>>> class A(object):
...     pass
... 
>>> A.injected = injected
>>> a = A()
>>> a.injected()
injected
>>> def func2(self):
...     print 'func2'
... 
>>> setattr(A, 'injected2', func2)
>>> a.injected2()
func2
>>>

answered Dec 29, 2012 at 3:08

warvariuc

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6 Comments

tadasajon Over a year ago

I don't follow. Your answer does not have a function name that is dynamically determined based on a string. Injecting the function isn't the hard part for me. Knowing how to define a function with a named based on a string is the part I don't understand.

warvariuc Over a year ago

I think the answer has all the necessary bits to accomplish what you need. You have a class A in which you want to inject a new method. Extract method name from a file, define any function (name doesn't matter) with the needed code, use setattr(A, method_name, your_defined_function) to inject a new method.

warvariuc Over a year ago

Functions in Python are first class citizens. Function name doesn't really matter and you can assign the same function to several variables. So your "Knowing how to define a function with a named based on a string" doesn't make sense for me. Define any function, then inject it under a different name into your class.

tadasajon Over a year ago

thanks. Your example seems to be working. I agree with you that function names should not usually matter, but I'm using Python unittest - it runs every function with a name that starts with "test" as a separate unit test, so it seems that the function name is important for using unittest.

warvariuc Over a year ago

Ok, so do something like setattr(TestCase, 'test_' + test_function_name, defined_function)

|

9000 · Accepted Answer · 2012-12-29 03:55:04Z

You don't need to define a function under one true name. Functions are first-class entitiens, you can pass them around and assign to variables. On top level, you use globals(), withing another function, locals() to bind a name:

>>> def foo(x):
...   return x + 1
... 
>>> name = 'unpredictable_words'
>>> 
>>> globals()['test_' + name] = foo
>>> 
>>> test_unpredictable_words(1)
2
>>> 
>>> def some_function():
...   locals()['test_' + name] = foo
...   return test_unpredictable_words(1)
... 
>>> some_function()
2
>>>

Sometimes you still want that the function knows its name, in order to appear nicely in a stacktrace. Now test_unpredictable_words appear as foo in error messages. This is not easy to fix, since that name is stored in foo.func_code.co_name and can't be changed.

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