Common While Loop Errors

3.8. Common While Loop Errors#

3.8.1. Infinite While Loop#

One of the easiest mistakes to make with a while loop is having a condition that is always True. This results in an infinite while loop!

For example:

i = 0
while i < 3:
    print(i)

What’s happening here?

We can see that the condition we have set for our while loop is i < 3.

At the first iteration i = 0. This means the condition is True. We execute the code block:

print(i)

This means that we output:

At the second iteration i = 0. This is because we did not update i. This means the condition is True. Again, we execute the code block and we output:

At the third iteration i = 0, at the fourth iteration i = 0, and so on. This is because we never updated i. This results in the loop never ending!

3.8.2. Not initialising your iteration variable#

If you forget to initialise your iteration variable you’ll get a NameError (A variable of the specified name cannot be found).

See what happens when you run the following code.

while i < 3:
    print(i)
    i = i + 1

The code above produces the following error message.

Traceback (most recent call last):
   File "/home/main.py", line 1, in <module>
      while i < 3:
            ^
   NameError: name 'i' is not defined. Did you mean: 'id'?

The error message tells us the error is on line 1 and comes from the while i < 3:.
It also tells us that we have a NameError, which means a variable hasn’t been defined. In this case it’s the variable i. The program is trying to check whether i is less than 3, but i does not exist.

3.8.3. Incorrect indentation#

It’s important that you get the indentation correct. Anything that is inside the loop, i.e. the code that is repeated, should be indented.

If nothing is indented you will get an IndentationError (The indentation is not syntactically valid).

i = 0
while i < 3:
print(i)
i = i + 1

 File "/home/main.py", line 3
   print(i)
   ^
IndentationError: expected an indented block after 'while' statement on line 2

You can also get an IndentationError if your indentation isn’t consistent. In the example below line 3 has 5 spaces in front of it while line 4 only has 4 spaces in front of it. The number of spaces in front of these lines need to be the same.

i = 0
while i < 3:
    print(i)
   i = i + 1

  File "/home/main.py", line 4
   i = i + 1
            ^
IndentationError: unindent does not match any outer indentation level

Common While Loop Errors

Contents

3.8. Common While Loop Errors#

3.8.1. Infinite While Loop#

3.8.2. Not initialising your iteration variable#

3.8.3. Incorrect indentation#