Open the file in vim or vi, and administer the following command:
:set ff=unix
Save and exit:
:wq
Done!
Explanation
ff stands for file format, and can accept the values of unix (\n), dos (\r\n) and mac (\r) (only meant to be used on pre-intel macs, on modern macs use unix).
To read more about the ff command:
:help ff
:wq stands for Write and Quit, a faster equivalent is Shift+zz (i.e. hold down Shift then press z twice).
Both commands must be used in command mode.
😱 offtopic: if by chance you are stuck in vim and need to exit, here are some easy ways.
Usage on multiple files
It is not necessary to actually open the file in vim. The modification can be made directly from the command line:
vi +':wq ++ff=unix' file_with_dos_linebreaks.py
To process multiple *.py files (in bash):
for file in *.py ; do
vi +':w ++ff=unix' +':q' "${file}"
done
While it looks innocent, the bash syntax above will break on file names containing spaces, quotes, dashes, etc, a more reliable alternative that will also process subfolders would be:
find . -iname '*.py' -exec vi +':w ++ff=unix' +':q' {} \;
Removing the BOM mark
Sometimes even after setting unix line endings you might still get an error running the file, especially if the file is executable, has a shebang, and you are running it without prefixing it with python. The script might have a BOM marker (such as 0xEFBBBF or other) which makes the shebang invalid and causes the shell to complain. In these cases python myscript.py will work fine (since python can handle the BOM) but ./myscript.py will fail when the execution bit is set because your shell (sh, bash, zsh, etc) can’t handle the BOM mark. (It’s usually windows editors such as Notepad that create files with a BOM mark.)
The BOM can be removed by opening the file in vim and administering the following command:
:set nobomb