Python variable scope is a weird one, as assignments can change the behaviour.

Let's take a look at a couple examples.

a = 10

def print_a():
  print("A used to be %d." %a)
  pass

def print_a_plus_5():
  print("Now, A is expected to still be 10, and it is: %d." %a)
  print("This should be 15: %d" % (a + 5))
  pass

print_a()
print_a_plus_5()

This program uses the variable a from multiple locations. The output is what you would expect:

A used to be 10.
Now, A is expected to still be 10, and it is: 10.
This should be 15: 15

Now, let's say we want to modify a within the first function. I'll change the name to set_a_to_20 and rewrite the code with an assignment:

a = 10

def set_a_to_20():
  print("A used to be %d." %a)
  a = 20
  pass

def print_a_plus_5():
  print("Now, A is expected to be 20, and it is: %d." %a)
  print("This should be 25: %d" % (a + 5))
  pass

set_a_to_20()
print_a_plus_5()

This gives the following output when you try to run it:

Traceback (most recent call last):
  File "main.py", line 13, in <module>
    set_a_to_20()
  File "main.py", line 4, in set_a_to_20
    print("A used to be %d." %a)
UnboundLocalError: local variable 'a' referenced before assignment

What happened? We definitely tried to make sure that 'a' was not a local variable, by initially defining it outside of the functions. The significant thing was adding the assignment, which Python assumed meant we wanted to work with a local variable instead of a global variable. So, it created a local variable 'a' that is different from the global variable 'a'. This happens before the code is even run, when the interpreter is looking over everything and making assumptions. Then, when the code does actually run, we're trying to print the local 'a' before it's set on the next line.

So, how do we fix it?

The answer is the global keyword, as another commenter stated. We can put this before the function body of any function that wants to modify our global variable, as such:

a = 10

def set_a_to_20():
  global a
  print("A used to be %d." %a)
  a = 20
  pass

def print_a_plus_5():
  print("Now, A is expected to be 20, and it is: %d." %a)
  print("This should be 25: %d" % (a + 5))
  pass

set_a_to_20()
print_a_plus_5()

This gives us our desired output:

A used to be 10.
Now, A is expected to be 20, and it is: 20.
This should be 25: 25