Thread by @treyhunner

I recommend against using the file readlines method in #Python.

Many folks assume that readlines returns a lazy iterable (an iterator) of lines. But the readlines method returns a list, not an iterator!

These are equivalent:

lines = my_file.readlines()
lines = list(my_file)

If you need to lazily loop over lines in a file, you can loop over the file object itself to do that:

for line in my_file:
...

You can use a file like any other iterable. For example you can use it in a comprehension:

non_empty = [line for line in my_file if line.strip()]

When I need to make a list of all lines in a file, I use the list constructor:

lines = list(my_file)

I find that clearer than this:

lines = my_file.readlines()

☝ That second line can lead newer Pythonistas to assume this is okay:

for line in my_file.readlines():
...

Just a reminder that you shouldn't do this:

for line in my_file.readlines():
...

That unnecessarily makes a list of all lines that gets deleted after the loop.

This does NOT store the whole file in memory, but loops over lines lazily instead:

for line in my_file:
...

More on reading files line-by-line in Python: pym.dev/reading-files-…

More on iterables in Python: pym.dev/terms/#iterable

Python Morsels subscribers can watch my whole series on working with files here: pym.dev/topics/playlis…