In Python, interacting with the file system is built right into the language.
The open() function returns a file object you can read from or write to. If the file can’t be opened — for example, it doesn’t exist — Python raises an OSError (in practice a specific subclass like FileNotFoundError):
try:
f = open('does_not_exist.txt')
except OSError as e:
print(f'Error: {type(e).__name__}')Error: FileNotFoundError
with StatementAlways open files with a with statement. It guarantees the file is closed automatically when the block ends — even if an error happens inside — so you never leak an open file handle or lose unsaved data:
with open('temp.txt', 'w') as f:
f.write('test')
print(f'Is file closed? {f.closed}')Is file closed? True
The second argument to open() is the mode. It defaults to 'r' — open for reading in text mode — so if you only want to read, you can leave it out:
with open('temp.txt', 'w') as f:
f.write('Default mode test')
with open('temp.txt') as f:
print(f.read())Default mode test
There are two ways to write. 'w' (write) truncates the file — it wipes any existing contents the moment you open it — while 'a' (append) keeps what’s there and adds to the end. Reach for 'w' to start fresh and 'a' to add on:
with open('temp.txt', 'w') as f:
f.write('Hello World!')
with open('temp.txt', 'r') as f:
print(f.read())Hello World!
You can also append to an existing file:
with open('temp_append.txt', 'w') as f:
f.write('First.')
with open('temp_append.txt', 'a') as f:
f.write(' Second.')
with open('temp_append.txt', 'r') as f:
print(f.read())First. Second.
To read a file line by line, just loop over the file object directly. This is the fastest and most memory-efficient way — Python reads one line at a time instead of loading the whole file into memory at once:
with open('temp_lines.txt', 'w') as f:
f.write('line1\nline2\nline3')
with open('temp_lines.txt') as f:
for line in f:
print(line.strip())line1 line2 line3
f.write().encoding='utf-8'.You don't need to close a file if your script ends soon anyway.
While the OS usually cleans up, relying on it is a bad habit that leads to resource leaks in larger programs. Always use `with open(...)`. — While the OS usually cleans up, relying on it is a bad habit that leads to resource leaks in larger programs. Always use `with open(...)`.
`f.read()` is always the best way to read a file.
If the file is massive, `f.read()` will load the entire thing into memory, potentially crashing your program. Looping over the file object is safer. — If the file is massive, `f.read()` will load the entire thing into memory, potentially crashing your program. Looping over the file object is safer.
`w` mode will safely add text to an existing file.
`w` mode completely truncates (erases) the file before writing. Use `a` (append) mode to add text to the end. — `w` mode completely truncates (erases) the file before writing. Use `a` (append) mode to add text to the end.
File objects return lines without the newline character.
File iteration and `readline()` leave the trailing `\n` intact. You typically need to call `.strip()` to remove it. — File iteration and `readline()` leave the trailing `\n` intact. You typically need to call `.strip()` to remove it.
Write a script that opens `message.txt` in read mode, reads the first 5 characters, and prints them.
# Setup file
with open('message.txt', 'w') as f:
f.write('Python is awesome')
# Open message.txt in read mode and print the first 5 characters
with open('message.txt', 'r') as f:
print(f.read(5))Append the string ' Done.' to the end of `log.txt`.
# Setup file
with open('log.txt', 'w') as f:
f.write('Task 1.')
# Append ' Done.' to log.txt
with open('log.txt', 'a') as f:
f.write(' Done.')
# Verify
with open('log.txt', 'r') as f:
print(f.read())