Open files use resources and may be locked, preventing other programs from using them. Anyway, it is good practice to use with when reading files, as it takes care of closing the file for you.
with open('file', 'r') as f:
read_data = f.read()
Here's an example of something "bad" that might happen if you leave a file open.
Open a file for writing in your python interpreter, write a string to it, then open that file in a text editor. On my system, the file will be empty until I close the file handle.
