Python: modify method-local variable inside inner method

Question

I'm writing a test method called, say, test_foo (using pytest). I am testing the behavior of a function, foo, which takes as an argument another function, get. foo calls get iteratively, conditionally on the return value of get, e.g.:

def foo(get, param): max_num_tries = 3 curr_num_tries = 0 response = get(param) while curr_num_tries < max_num_tries and response.status_code == BAD_STATUS_CODE: response = get(param) return response

I am trying to override get such that it has access to how many times it's been called and can return different values accordingly.

Here's a simplified version of what I have so far:

def test_foo(): tries_so_far = 0 def get(arg1): global tries_so_far if tries_so_far < 3: tries_so_far += 1 print("do something special here") else: print("do something else here") return "some return val" foo(get, "some arg")

However, I get the following error:

NameError: global name 'tries_so_far' is not defined

If I define tries_so_far outside of test_foo, at the module level, I get the expected behavior. However, I would like tries_so_far to be a variable that is local to test_foo.

Is there some way to give get read/write to tries_so_far using globals, or some other technique? Note: I cannot change the parameters or return value of get.

You do not appear to be using write access to tries_so_far. Just remove the global and it will be able to read the variable. global means "module level", as you have correctly determined. — Mad Physicist
– Mad Physicist, Commented Feb 12, 2016 at 17:13
I am assuming you are using Py3k from the print statements. If that is true, can you change the tag? — Mad Physicist
– Mad Physicist, Commented Feb 12, 2016 at 17:19
Does this answer your question? Is it possible to modify variable in python that is in outer, but not global, scope? and UnboundLocalError on local variable when reassigned after first use — Karl Knechtel
– Karl Knechtel, Commented Sep 10, 2022 at 10:38

Community · Accepted Answer · 2017-05-23 12:31:08Z

According to the accepted answer for this question Why can functions in Python print variables in enclosing scope but cannot use them in assignment?, there is an additional statement added in Python 3: nonlocal that does what you want. It is like global, but says to look at the enclosing scope instead of the module level. The following modification should therefore allow you to do exactly what you want:

def test_foo(): tries_so_far = 0 def get(arg1): nonlocal tries_so_far if tries_so_far < 3: tries_so_far += 1 print("do something special here") else: print("do something else here") return "some return val" foo(get, "some arg")

While your question is not an exact duplicate of the one I cited above, you should read the accepted answer. It is very good and will probably address many unstated questions that you have on the subject.

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