TypeError/IndexError when iterating with a for loop, and referencing lst[i]

问题

I'm using a for loop to iterate over a list like this:

lst = ['a', 'b', 'c']
for i in lst:
    print(lst[i])

But there must be something wrong with that, because it throws the following exception:

Traceback (most recent call last):
  File "untitled.py", line 3, in <module>
    print(lst[i])
TypeError: list indices must be integers or slices, not str

And if I try the same thing with a list of integers, it throws an IndexError instead:

lst = [5, 6, 7]
for i in lst:
    print(lst[i])

Traceback (most recent call last):
  File "untitled.py", line 4, in <module>
    print(lst[i])
IndexError: list index out of range

What's wrong with my for loop?

回答1:

Python's for loop iterates over the values of the list, not the indices:

lst = ['a', 'b', 'c']
for i in lst:
    print(i)

# output:
# a
# b
# c

That's why you get an error if you try to index lst with i:

>>> lst['a']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: list indices must be integers or slices, not str

>>> lst[5]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range

Many people use indices to iterate out of habit, because they're used to doing it that way from other programming languages. In Python you rarely need indices. Looping over the values is much more convenient and readable:

lst = ['a', 'b', 'c']
for val in lst:
    print(val)

# output:
# a
# b
# c

And if you really need the indices in your loop, you can use the enumerate function:

lst = ['a', 'b', 'c']
for i, val in enumerate(lst):
    print('element {} = {}'.format(i, val))

# output:
# element 0 = a
# element 1 = b
# element 2 = c

回答2:

Corollary: name your loop-variable to avoid confusion and bad code

for i in lst is a TERRIBLE name
- suggests it's an index, and that we can and should do lst[i], which is nonsense, and throws error
- names like i, j, n are typically only used for indices
GOOD: for x in lst, for el in lst, for lx in lst.
- Noone would ever try to write lst[el]; the choice of name makes very obvious it ain't no index, and protects you from writing nonsense.

Summary:

Python for-loop variables assume values from the list, not its indices
usually you don't need the indices, but if you do, use enumerate() : for i, x in enumerate(list): ...
usually better idiom to directly iterate over the list (not its indices, and lookup each entry)

来源：https://stackoverflow.com/questions/52890793/typeerror-indexerror-when-iterating-with-a-for-loop-and-referencing-lsti

标签

python

loops

for-loop

typeerror

index-error