how to count the total number of lines in a text file using python

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You can use sum() with a generator expression:

with open('data.txt') as f:
    print sum(1 for _ in f)

Note that you cannot use len(f), since f is an iterator. _ is a special variable name for throwaway variables, see What is the purpose of the single underscore “_” variable in Python?.

You can use len(f.readlines()), but this will create an additional list in memory, which won’t even work on huge files that don’t fit in memory.

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