Python using enumerate inside list comprehension

Question

Lets suppose I have a list like this:

mylist = ["a","b","c","d"] 

To get the values printed along with their index I can use Python's enumerate function like this

>>> for i,j in enumerate(mylist): ...     print i,j ... 0 a 1 b 2 c 3 d >>> 

Now, when I try to use it inside a list comprehension it gives me this error

>>> [i,j for i,j in enumerate(mylist)]   File "<stdin>", line 1     [i,j for i,j in enumerate(mylist)]            ^ SyntaxError: invalid syntax 

So, my question is: what is the correct way of using enumerate inside list comprehension?

Answer 1

Try this:

[(i, j) for i, j in enumerate(mylist)] 

You need to put i,j inside a tuple for the list comprehension to work. Alternatively, given that enumerate() already returns a tuple, you can return it directly without unpacking it first:

[pair for pair in enumerate(mylist)] 

Either way, the result that gets returned is as expected:

> [(0, 'a'), (1, 'b'), (2, 'c'), (3, 'd')] 

Answer 2

Just to be really clear, this has nothing to do with enumerate and everything to do with list comprehension syntax.

This list comprehension returns a list of tuples:

[(i,j) for i in range(3) for j in 'abc'] 

this a list of dicts:

[{i:j} for i in range(3) for j in 'abc'] 

a list of lists:

[[i,j] for i in range(3) for j in 'abc'] 

a syntax error:

[i,j for i in range(3) for j in 'abc'] 

Which is inconsistent (IMHO) and confusing with dictionary comprehensions syntax:

>>> {i:j for i,j in enumerate('abcdef')} {0: 'a', 1: 'b', 2: 'c', 3: 'd', 4: 'e', 5: 'f'} 

And a set of tuples:

>>> {(i,j) for i,j in enumerate('abcdef')} set([(0, 'a'), (4, 'e'), (1, 'b'), (2, 'c'), (5, 'f'), (3, 'd')]) 

As Óscar López stated, you can just pass the enumerate tuple directly:

>>> [t for t in enumerate('abcdef') ]  [(0, 'a'), (1, 'b'), (2, 'c'), (3, 'd'), (4, 'e'), (5, 'f')]