python - Why does printing a variable in global scope work, but modifying it does not? -

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x = 1 def fn():     print x fn()

this prints "1":

x = 1 def fn():     x += 1     print x fn()

this raises "unboundlocalerror: local variable 'x' referenced before assignment"

what going on here?

in python, assigning variable implicit local declaration, resolved during bytecode compilation. so

x += 1

will create local variable x , compile byte code: 

0 load_fast                0 (x) 3 load_const               1 (1) 6 inplace_add 7 store_fast               0 (x)

the command load_fast try load local variable x not yet defined, that's why fails.

however, if define x global explicitly, use load_global/store_global instead.

in case of print in first function, compiler assumes since no local variable declared (assigned) ever in function body, should mean global variable.


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