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TypeError: list indices must be integers or slices, not str

What it means

You put something other than a whole number inside the square brackets of a list. Lists are numbered 0, 1, 2 and so on, so items[0] works, but items['name'] or items[1.5] does not. The last word of the message tells you what you used instead.

1. Using the loop item as if it were a position

This raises the error

Broken
fruits = ['apple', 'banana', 'cherry']
for fruit in fruits:
print(fruits[fruit])
Output
Traceback (most recent call last):
  File "<exec>", line 3, in <module>
TypeError: list indices must be integers or slices, not str

The fix

Fixed
fruits = ['apple', 'banana', 'cherry']
for fruit in fruits:
print(fruit) # the loop already gives you the item
for i, fruit in enumerate(fruits):
print(i, fruit) # enumerate() when you need the position too
Output
apple
banana
cherry
0 apple
1 banana
2 cherry

for fruit in fruits hands you each item, not its position, so fruit is already 'apple'. enumerate() gives you both the position and the item.

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2. A position worked out with `/` is a float

This raises the error

Broken
nums = [3, 8, 1, 9, 4]
middle = nums[len(nums) / 2]
print(middle)
Output
Traceback (most recent call last):
  File "<exec>", line 2, in <module>
TypeError: list indices must be integers or slices, not float

The fix

Fixed
nums = [3, 8, 1, 9, 4]
middle = nums[len(nums) // 2] # // keeps it a whole number
print(middle)
Output
1

/ always returns a float, even 4 / 2 gives 2.0. Floor division // returns an int, which works as a list index.

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3. Treating a list of dictionaries as one dictionary

This raises the error

Broken
users = [{'name': 'Ana', 'age': 31}, {'name': 'Ben', 'age': 25}]
print(users['name'])
Output
Traceback (most recent call last):
  File "<exec>", line 2, in <module>
TypeError: list indices must be integers or slices, not str

The fix

Fixed
users = [{'name': 'Ana', 'age': 31}, {'name': 'Ben', 'age': 25}]
print(users[0]['name']) # first pick a dictionary, then a key
for user in users:
print(user['name'], user['age'])
Output
Ana
Ana 31
Ben 25

The outer list needs a number to choose one dictionary, and only then does a key like 'name' work. Data from JSON often has this shape.

Try fixing it yourself in the editor

When you see this error, check

  • Print the value inside the brackets and its type().
  • Use // instead of / when you work out a position.
  • Convert text to a number with int() before using it as an index.
  • If you want to look things up by name, use a dictionary instead of a list.

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