Python File Handling

1Python File Handling: overview

File handling means using open() to connect your program to a file, reading text from it or writing text into it, and then closing it so the work is saved.

Before this lesson: Python Strings, Python for Loops, Python Exceptions: try, except, else and finally

2Python File Handling: diagram

Quick reference: Python File Handling

• Python File Handling Cheat Sheet: open, read, write, CSV, JSON — Open files with `with open(path, mode, encoding="utf-8") as f:` so they close automatically, read with `read(…
• How to Read a File Line by Line in Python — Open the file with `with open("file.txt") as f:` and loop over it: `for line in f:`. Each `line` is one line …
• How to Write to a File in Python — Use `with open("notes.txt", "w", encoding="utf-8") as f:` then `f.write("text\n")`. Mode `"w"` replaces the f…
• How to Read a CSV File in Python — Use the `csv` module: `with open("data.csv", newline="", encoding="utf-8") as f:` then `csv.DictReader(f)` gi…
• How to Read a JSON File in Python — Open the file and call `json.load()`: `with open("data.json", encoding="utf-8") as f: data = json.load(f)`. Y…
• How to Check if a File Exists in Python — Use `pathlib`: `Path("data.txt").is_file()` is `True` only if the path exists and is a regular file. `Path(..…
• How to Convert JSON to a Dictionary in Python — Use `json.loads(text)` to turn a JSON string into a dictionary, or `json.load(file)` to read it from an open …
• Python String rstrip() Method — `text.rstrip(chars=None)` returns a new string with **trailing** (right-hand) whitespace, or any characters i…

4Python File Handling: syntax

with open("file.txt", "w") as f:   # "w" write (overwrite), "a" append
    f.write("some text\n")         # strings only; add \n yourself

with open("file.txt") as f:        # "r" (read) is the default mode
    text = f.read()                # whole file as one string
    # lines = f.readlines()        # list of lines
    # for line in f: ...           # one line at a time

5Python File Handling: run the example

Press Run (or Ctrl + Enter). The output (step 6) appears directly below the code. Change anything and run it again.

entries = ["Mon: 5 km run", "Wed: 30 min swim", "Fri: 8 km run"]

with open("workouts.txt", "w") as f:
    for entry in entries:
        f.write(entry + "\n")

with open("workouts.txt", "a") as f:
    f.write("Sun: yoga\n")

with open("workouts.txt") as f:
    lines = f.readlines()

print("File closed:", f.closed)
print("Lines in file:", len(lines))
for number, line in enumerate(lines, start=1):
    print(number, line.strip())

Expected outputFile closed: True
Lines in file: 4
1 Mon: 5 km run
2 Wed: 30 min swim
3 Fri: 8 km run
4 Sun: yoga

7Python File Handling: how the example works

Hover or tap a step to highlight its lines in the example above.

1. Line 1A list of three workout notes is created. This is the data we want to save.
2. Line 3open("workouts.txt", "w") creates the file (or empties it if it already exists) and names the file object f.
3. Line 4-5Each entry is written with "\n" added, because write() does not start a new line by itself.
4. Line 7-8Mode "a" keeps the three existing lines and adds "Sun: yoga" at the end.
5. Line 10-11With no mode given, the file opens for reading. readlines() returns a list of 4 strings, each still ending in "\n".
6. Line 13We are outside the with block now, so f.closed is True — Python closed the file automatically.
7. Line 14len(lines) counts the lines that came back from the file: 4.
8. Line 15-16enumerate(..., start=1) numbers the lines from 1, and strip() removes the trailing newline so print() does not add blank lines.

8Python File Handling: common mistakes

with open("diary.txt") as f:
    print(f.read())

try:
    with open("diary.txt") as f:
        print(f.read())
except FileNotFoundError:
    print("No diary yet - creating one.")
    with open("diary.txt", "w") as f:
        f.write("Day 1\n")

with open("log.txt", "w") as f:
    f.write("first visit\n")
with open("log.txt", "w") as f:
    f.write("second visit\n")
with open("log.txt") as f:
    print(f.read())

with open("log.txt", "w") as f:
    f.write("first visit\n")
with open("log.txt", "a") as f:
    f.write("second visit\n")
with open("log.txt") as f:
    print(f.read())

score = 95
with open("best.txt", "w") as f:
    f.write(score)

score = 95
with open("best.txt", "w") as f:
    f.write(f"{score}\n")

9Python File Handling: exercises

Write your answer, then press Check. The checks run in your browser.

def save_lines(filename, lines):
    # open the file in write mode and write each line followed by "\n"
    pass

save_lines("shopping.txt", ["eggs", "rice", "apples"])

def save_lines(filename, lines):
    with open(filename, "w") as f:
        for line in lines:
            f.write(line + "\n")

save_lines("shopping.txt", ["eggs", "rice", "apples"])

with open("pets.txt", "w") as f:
    f.write("cat\n")
with open("pets.txt", "a") as f:
    f.write("dog\n")
with open("pets.txt", "w") as f:
    f.write("fish\n")
with open("pets.txt", "a") as f:
    f.write("bird\n")

with open("pets.txt") as f:
    print(f.read().split())

['fish', 'bird']

2 more exercises → 4 more examples

Further reading

• Input and output
• itertools
• Functional programming HOWTO
• pathlib

10Next lesson

Related:
Python with StatementPython pathlibWorking with JSON in PythonPython CSV Files

← Python itertools · All Iteration and Files lessons · Practise in the IDE