python - Must I always reference a variable as global in my function if I were to use it? -

firstly, it's related function:

a = 3 def b():     = a+1     return  

returns error.

isn't when local environment of b couldn't find 'a', goes parent environment b defined, is, 'a' found 3?

why must reference global a in order work? meaning, have when immediate parent environment global environment? because below function seems able reference parent environment:

def b():     def c():         print c     return c() 

this such common issue is in python faq:

[...] when make assignment variable in scope, variable becomes local scope , shadows named variable in outer scope. since last statement in foo assigns new value x, compiler recognizes local variable.

so, python sees you're assigning local variable a = .. , thinks, hey, local variable, i'm fine this. but, when bumps content of assignment (a + 1) , sees same name a complains because you're trying reference before assigning it.

this why following doesn't raise error:

def b():     = 20     = + 2     return 

you first make assignment, a treated local function b , when a = + 2 encountered, python knows a local variable, no confusion present here.

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in other case added:

def b():     def c():         print c     return c() 

you referencing name enclosed in scope of function b, referencing. if changed similar assignment before, you'll same unboundlocalerror:

def b():     def c():         c = c + 20     return c()