CodeQL documentation

Potentially uninitialized local variable

ID: py/uninitialized-local-variable
Kind: problem
Severity: error
Precision: medium
Tags:
   - reliability
   - correctness
Query suites:
   - python-security-and-quality.qls

Click to see the query in the CodeQL repository

This local variable may be used before it is defined. If a variable is assigned to in a function and not explicitly declared global or nonlocal then it is assumed to be a local variable. If it is used before it is defined then an UnboundLocalError will be raised.

Recommendation

Review the code and consider the intended scope of the variable. Determine whether the variable should be global or local in scope. If a global variable is required then add a global statement, or in Python 3 you can use a nonlocal statement if the variable occurs in an enclosing function. Otherwise, ensure that the variable is defined before it is used.

Example

The following code includes different functions that use variables. test1() fails with an UnboundLocalError because the local variable var is used before it is initialized.

def test():
    var = 1 
    def print_var():
        print var      # Use variable from outer scope
    print_var()
    print var 


def test1():
    var = 2 
    def print_var():
        print var       # Attempt to use variable from local scope. 
        var = 3         # Since this is not initialized yet, this results
    print_var()         # in an UnboundLocalError
    print var 


def test2():
    var = 2 
    def print_var():
        var = 3         # Initialize local version of the variable
        print var       # Use variable from local scope.
    print_var()         # Note that this local variable "shadows" the variable from
    print var           # outer scope which makes code more difficult to interpret.


def test3():
    var = 4
    def print_var():
        nonlocal var    # Use non-local variable from outer scope.
        print var
    print_var()
    print var

References