File Handling in Python

Text files with open() modes, read, readline, readlines, write, seek and tell; binary files with pickle; and CSV files with reader and writer.

Key ideas

• open(name, mode) gives a file object; the default mode is 'r', and close() (or a with block) saves and frees the file.
• 'w' creates the file or empties it, 'a' writes at the end, 'r' needs the file to exist; + adds reading or writing, and b makes it a binary file.
• read() gives the rest of the file as one string, read(n) the next n characters, readline() one line with its newline, readlines() a list of lines.
• write(s) writes a string and adds no newline; writelines(L) writes each string of a list, also adding none.
• tell() gives the current position of the file pointer; seek(k) moves it to position k, counting from 0.
• pickle.dump(obj, f) stores a Python object in a binary file and pickle.load(f) reads one back; reading past the end raises EOFError.
• csv.writer(f).writerow(row) writes one line of comma-separated fields and writerows() many; csv.reader(f) gives each row as a list of strings.

Remember

Watch out

• Opening a file with 'w' to add to it: it is emptied first; use 'a'.
• Leaving out the newline characters when counting what read() gives.
• Using a value read from a CSV file as a number without int(): it is a string.
• Expecting write() to start a new line by itself.

Worked example