Variable error in Python

Question

I'm in a Python course and can't figure out why my code won't work:

import os
def rename_files():
    file_list = os.listdir(r"C:\Users\Kyle\Desktop\prank")

for file_name in file_list:
    os.rename(file_name, file_name.translate(None,"0123456789"))
rename_files()

The code above returns:

Traceback (most recent call last):
  File "C:/Users/Kyle/Desktop/renamepy.py", line 5, in <module>
    for file_name in file_list:
NameError: name 'file_list' is not defined
>>>

Why file_name is not defined?

Karin · Accepted Answer · 2016-08-22 02:35:04Z

2

This is an indentation error. Your for loop is outside of the scope of rename_files() since it was not indented. It should be:

import os
def rename_files():
    file_list = os.listdir(r"C:\Users\Kyle\Desktop\prank")

    for file_name in file_list:
        os.rename(file_name, file_name.translate(None,"0123456789"))

rename_files()

answered Aug 22, 2016 at 2:35

Karin

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Comments

alvas · Accepted Answer · 2016-08-22 02:51:43Z

Let's walk-through your code:

def rename_files():
    file_list = os.listdir(r"C:\Users\Kyle\Desktop\prank")

In you rename_files() function, you have defined a function that returns nothing. You have set a local scoped file_list that will be released and cannot be accessed once you're outside the function.

for file_name in file_list:
    os.rename(file_name, file_name.translate(None,"0123456789"))
rename_files()

Then when you're outside of the rename_files() function, you try to access a file_list that has not previously been initialized in the for-loop, so it throws the NameError

NameError: name 'file_list' is not defined

And then you called the rename_files() function but still file_list will not exist outside of the function.

There's several ways to ensure that file_list is materialized before you go through the for-loop.

Solution 1: Use the global variable.

file_list = []

def rename_files():
    global file_list
    file_list = os.listdir(r"C:\Users\Kyle\Desktop\prank")
rename_files()
for file_name in file_list:
    os.rename(file_name, file_name.translate(None,"0123456789"))

Solution 2: take the file_list initialization outside of the function.

file_list = os.listdir(r"C:\Users\Kyle\Desktop\prank")
for file_name in file_list:
    os.rename(file_name, file_name.translate(None,"0123456789"))

Solution 3 (as suggested by Karin): Put the for-loop into the function:

def rename_files():
    file_list = os.listdir(r"C:\Users\Kyle\Desktop\prank")
    for file_name in file_list:
        os.rename(file_name, file_name.translate(None,"0123456789"))

Note: this is similar to Solution 2 since they are trying to put both the file_list initialization and the for-loop under the same scope.

Thank you! I am used to brackets. I didn't know indentation defines the scope!

Burhan Khalid · Accepted Answer · 2016-08-22 02:35:21Z

0

You need to indent your for loop so its within the body of your rename_file function:

import os

def rename_files():
    file_list = os.listdir(r"C:\Users\Kyle\Desktop\prank")

    for file_name in file_list:
        os.rename(file_name, file_name.translate(None,"0123456789"))

rename_files()

As file_list is only in scope within the method.

answered Aug 22, 2016 at 2:35

Burhan Khalid

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