How to print the progress of a list comprehension in python?

Question

In my method i have to return a list within a list. I would like to have a list comprehension, because of the performance since the list takes about 5 minutes to create.

Answer 1

I have the need to make @ted's answer (imo) more readable and to add some explanations.

Tidied up solution:

# Function to print the index, if the index is evenly divisable by 1000:
def report(index):
    if index % 1000 == 0:
        print(index)

# The function the user wants to apply on the list elements
def process(x, index, report):
     report(index) # Call of the reporting function
     return 'something ' + x # ! Just an example, replace with your desired application

# !Just an example, replace with your list to iterate over
mylist = ['number ' + str(k) for k in range(5000)]

# Running a list comprehension
[process(x, index, report) for index, x in enumerate(mylist)]

Explanation: of enumerate(mylist): using the function enumerate it is possible to have indices in addition to the elements of an iterable object (cf. this question and its answers). For example

[(index, x) for index, x in enumerate(["a", "b", "c"])] #returns
[(0, 'a'), (1, 'b'), (2, 'c')]

Note: index and x are no reserved names, just names I found convenient - [(foo, bar) for foo, bar in enumerate(["a", "b", "c"])] yields the same result.