Setup and Testing
To analyze your question, let's create two separate test functions that replicate your issue:
a=0
def test1():
print(a)
test1()
prints 0
. So, calling this function is not problematic, but on the next function:
def test2():
print(a) # Error : local variable 'a' referenced before assignment
a=0
test2()
We get an error:
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 2, in test2
UnboundLocalError: local variable 'a' referenced before assignment
Disassembly
We can disassemble the two functions (first Python 2):
>>> import dis
>>> dis.dis(test1)
2 0 LOAD_GLOBAL 0 (a)
3 PRINT_ITEM
4 PRINT_NEWLINE
5 LOAD_CONST 0 (None)
8 RETURN_VALUE
And we see that the first function automatically loads the global a
, while the second function:
>>> dis.dis(test2)
2 0 LOAD_FAST 0 (a)
3 PRINT_ITEM
4 PRINT_NEWLINE
3 5 LOAD_CONST 1 (0)
8 STORE_FAST 0 (a)
11 LOAD_CONST 0 (None)
14 RETURN_VALUE
seeing that a
is assigned inside it, attempts to LOAD_FAST from the locals (as a matter of optimization, as functions are pre-compiled into byte-code before running.)
If we run this in Python 3, we see nearly the same effect:
>>> test2()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 2, in test2
UnboundLocalError: local variable 'a' referenced before assignment
>>>
>>> import dis
>>> dis.dis(test1)
2 0 LOAD_GLOBAL 0 (print)
3 LOAD_GLOBAL 1 (a)
6 CALL_FUNCTION 1 (1 positional, 0 keyword pair)
9 POP_TOP
10 LOAD_CONST 0 (None)
13 RETURN_VALUE
>>> dis.dis() # disassembles the last stack trace
2 0 LOAD_GLOBAL 0 (print)
--> 3 LOAD_FAST 0 (a)
6 CALL_FUNCTION 1 (1 positional, 0 keyword pair)
9 POP_TOP
3 10 LOAD_CONST 1 (0)
13 STORE_FAST 0 (a)
16 LOAD_CONST 0 (None)
19 RETURN_VALUE
We see our error is on the LOAD_FAST again.