How to get the line count of a large file cheaply in Python

One line, faster than the for loop of the OP (although not the fastest) and very concise:

num_lines = sum(1 for _ in open('myfile.txt'))

You can also boost the speed (and robustness) by using rbU mode and include it in a with block to close the file:

with open("myfile.txt", "rbU") as f:
    num_lines = sum(1 for _ in f)

Note: The U in rbU mode is deprecated since Python 3.3 and above, so iwe should use rb instead of rbU (and it has been removed in Python 3.11).