Doubts about python variable scope [duplicate]

Question

Possible Duplicate:
Short Description of Python Scoping Rules

I wrote two simple functions:

# coding: utf-8
def test():
    var = 1 
    def print_var():
        print var 
    print_var()
    print var 

test()
# 1
# 1
def test1():
    var = 2 
    def print_var():
        print var 
        var = 3 
    print_var()
    print var 

test1()
# raise Exception

In comparison, test1() assigns value after print var, then raise an Exception: UnboundLocalError: local variable 'var' referenced before assignment, I think the moment I call inner print var, var has a value of 2, am I wrong?

@HansThen: No, a global declaration would not help here, as var is a function-scope variable. In python 3 you'd use nonlocal but in python 2 (as used here) there is no way you can do what the OP is trying to do. — Martijn Pieters
– Martijn Pieters, Commented Sep 12, 2012 at 6:53

wim · Accepted Answer · 2012-09-12 07:09:12Z

1

Yes, you're incorrect here. Function definition introduces a new scope.

# coding: utf-8
def test():
    var = 1 
    def print_var():
        print var    <--- var is not in local scope, the var from outer scope gets used
    print_var()
    print var 

test()
# 1
# 1
def test1():
    var = 2 
    def print_var():
        print var     <---- var is in local scope, but not defined yet, ouch
        var = 3 
    print_var()
    print var 

test1()
# raise Exception

edited Sep 12, 2012 at 7:09

answered Sep 12, 2012 at 6:43

wim

368k114 gold badges681 silver badges817 bronze badges

Sign up to request clarification or add additional context in comments.

7 Comments

Lauritz V. Thaulow Over a year ago

Hang on, I thought python did not look ahead when executing. How does python know that var is going to be a local variable in the second print_var function?

Lauritz V. Thaulow Over a year ago

So when python parses the function it saves a list of the names of all variables that are assigned to within. Then at call time it sees that var is in this list, and complains since it's not defined yet. Is this local variable list accessible?

Martijn Pieters Over a year ago

@lazyr: Exactly; variables are bound to a scope at compile time. Assignment is a storing operation, and binds that variable to the local scope unless marked global. If there is no assignment, the variable is 'free' and looked up in surrounding scopes, but this is bound as well to that specific scope. See Python nested scopes with dynamic features for example.

Lauritz V. Thaulow Over a year ago

@Martijn Can I access the scope of a function and see what its local variables are called?

Martijn Pieters Over a year ago

@lazyr: Yes, see docs.python.org/reference/datamodel.html, afunction.func_code.co_varnames are local variables. co_freevars and co_cellvars are interesting in this respect; they are variables taken from the surrounding scope (non-global) and variables referred to by nested functions, respectively.

|

Collectives™ on Stack Overflow

Doubts about python variable scope [duplicate]

1 Answer 1

7 Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

7 Comments

Linked

Related