For example if my text file is:
blue green yellow black
Here there are four lines and now I want to get the result as four. How can I do that?
You can use sum() with a generator expression:
sum()
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?.
len(f)
f
_
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.
len(f.readlines())
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