TypeError: list indices must be integers or slices, not tuple
Estimated reading time: 3 minutes
When working with Python lists in your data analytics projects, when you trying to read the data, a common problem occurs if you have a list of lists, and it is not properly formatted.
In this instance, Python will not be able to read one or more lists and as a result, will throw this error.
In order to understand how this problem occurs, we need to understand how to create a list of lists.
How to create a lists of lists
Let’s look at a simple list:
a = [1,2,3]
print(a)
print(type(a))
Result:
[1, 2, 3]
<class 'list'>Let’s create a second list called b:
b = [4,5,6]
print(b)
print(type(b))
Result:
[4, 5, 6]
<class 'list'>So if we want to join the lists together into one list ( hence a list of lists) then:
a = [1,2,3] b = [4,5,6] list_of_lists = [] list_of_lists.append(a) list_of_lists.append(b) print(list_of_lists) print(type(list_of_lists)) Result: [[1, 2, 3], [4, 5, 6]] <class 'list'>
So as can be seen the two lists are contained within a master list called “list_of_lists”.
So why does the error list indices must be integers or slices, not tuple occur?
Reason 1 ( Missing commas between lists)
If you manually type them in and forget the comma between the lists this will cause your problem:
a=[[1,2,3][4,5,6]]
print(a)
Result (Error):
Traceback (most recent call last):
line 10, in <module>
a=[[1,2,3][4,5,6]]
TypeError: list indices must be integers or slices, not tuple
But if you put a comma between the two lists then it returns no error:
a=[[1,2,3],[4,5,6]] print(a) Result (no error): [[1, 2, 3], [4, 5, 6]] Process finished with exit code 0
Reason 2 ( Comma in the wrong place)
Sometimes you have a list, and you only want to bring back some elements of the list, but not others:
In the below, we are trying to bring back the first two elements of the list.
a= [1,2,3,4,5]
print(a[0:,2])
Result:
Traceback (most recent call last):
line 14, in <module>
print(a[0:,2])
TypeError: list indices must be integers or slices, not tuple
The reason that the same error happens is the additional comma in a[0:,2], causes the error to appear as Python does not know how to process it.
This is easily fixed by removing the additional comma as follows:
a= [1,2,3,4,5] print(a[0:2]) Result: [1, 2] Process finished with exit code 0
So why is there a mention of tuples in the error output?
The final piece of the jigsaw needs to understand why there is a reference to a tuple in the error output?
If we return to a looking at a list of lists and look at their index values:
a=[[1,2,3],[4,5,6]]
z = [index for index, value in enumerate(a)]
print(z)
Result:
[0, 1]
Process finished with exit code 0As can be seen, the index values are 0,1, which is correct.
As before removing the comma gives the error we are trying to solve for:
a=[[1,2,3][4,5,6]]
z = [index for index, value in enumerate(a)]
print(z)
Result:
Traceback (most recent call last):
line 16, in <module>
a=[[1,2,3][4,5,6]]
TypeError: list indices must be integers or slices, not tuple
BUT the reference to the tuple comes from the fact that when you pass two arguments (say an index value) a Tuple is created, but in this instance, as the comma is missing the tuple is not created and the error is called.
This stems from the __getitem__ for a list built-in class cannot deal with tuple arguments that are not integers ( i.e. 0,1 like we have returned here) , so the error is thrown, it is looking for the two index values to pass into a tuple.