PythonFileIOExceptions

Python — File I/O and Exceptions

File I/O with pathlib

pathlib.Path is the modern way to work with files. It handles cross-platform paths and provides clean read_text() / write_text() methods.

Reading a file

from pathlib import Path
 
path     = Path('siddhartha.txt')
contents = path.read_text()         # entire file as one string
lines    = contents.splitlines()    # split into a list of lines
 
for line in lines:
    print(line)

Writing to a file

path = Path('journal.txt')
msg  = "I love programming."
path.write_text(msg)                # creates file if missing; overwrites if present

pathlib vs open()

The older style uses open()/f.read()/context managers. pathlib is the modern approach — simpler for basic read/write and handles OS path separators automatically. The cheat sheet uses pathlib exclusively.

Exceptions

Exceptions let you respond to errors gracefully rather than crashing. Structure:

try:
    [code that might raise an exception]
except SomeSpecificError:
    [what to do if that error occurs]
else:
    [runs only if NO exception was raised in try]

Catching a ValueError

prompt      = "How many tickets do you need? "
num_tickets = input(prompt)
 
try:
    num_tickets = int(num_tickets)    # raises ValueError if not numeric
except ValueError:
    print("Please try again.")
else:
    print("Your tickets are printing.")

Catch specific exceptions

Always name the specific exception (e.g., ValueError, FileNotFoundError) rather than a bare except:. Bare except: silently swallows unexpected bugs.

Cross-References

• PythonControlFlow — try/except is a branching control structure
• PythonVariablesStringsInput — exceptions most often arise when converting input() output
• CppIOStreams — contrast: C++ uses streams (cin/fstream) rather than path objects

graph TD
    A[File I/O] --> B["Path('filename.txt')"]
    B --> C["path.read_text() → string"]
    B --> D["path.write_text(msg)"]
    C --> E["contents.splitlines() → list of lines"]
    F[Exceptions] --> G["try: risky code"]
    G --> H["except ValueError: handle error"]
    G --> I["else: runs if no exception raised"]