UnboundLocalError
While the variable in the function has already been defined, during execution, the result prints with an error of UnboundLocalError. Below, you will find an example that explains the issue and resolution.
Error: file ‘example.txt’ not found
Traceback (most recent call last):
File “main.py”, line 16, in <module>
open_file(“example.txt”)
File “main.py”, line 11, in open_file
if f:
UnboundLocalError: local variable ‘f’ referenced before assignment
** Process exited — Return Code: 1 **
Press Enter to exit terminal
Python program using try, except and finally
Below program explains how to use try, except and finally blocks in python. But during the execution, it prints UnboundLocalError.
Program using try, except and finally
# Define a function that may raise an exception
def open_file(filename):
try:
f = open(filename, “r”)
contents = f.read()
except FileNotFoundError:
print(f”Error: file ‘{filename}’ not found”)
else:
print(f”Contents of file ‘{filename}’:\n{contents}”)
finally:
if f:
f.close()
print(f”File ‘{filename}’ has been closed.”)
# Call the function with a valid filename
open_file(“example.txt”)
# Call the function with an invalid filename
open_file(“invalid.txt”)
The output:
Error: file ‘example.txt’ not found
Traceback (most recent call last):
File “main.py”, line 16, in <module>
open_file(“example.txt”)
File “main.py”, line 11, in open_file
if f:
UnboundLocalError: local variable ‘f’ referenced before assignment
** Process exited — Return Code: 1 **
Press Enter to exit terminal
Resolution for UnboundLocalError
In the function, before the try block, add f = None. That resolves the issue. Here is the corrected program.
Corrected program
# Define a function that may raise an exception
def open_file(filename):
try:
f = None
f = open(filename, “r”)
contents = f.read()
except FileNotFoundError:
print(f”Error: file ‘{filename}’ not found”)
else:
print(f”Contents of file ‘{filename}’:\n{contents}”)
finally:
if f:
f.close()
print(f”File ‘{filename}’ has been closed.”)
# Call the function with a valid filename
open_file(“example.txt”)
# Call the function with an invalid filename
open_file(“invalid.txt”)
The output is:
Error: file ‘example.txt’ not found
Error: file ‘invalid.txt’ not found
** Process exited — Return Code: 0 **
Press Enter to exit terminal
Summary
- Return statement not coded in the function. As a result, it gave UnboundLocalError.
- A function returns the None object by default if the control flow falls off the end of the function body without running into a return statement.
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