Text Files#

Python provides built-in functions for reading and writing text files using file objects.

Creating a File Object#

Use the open() function to create a file object. Always use a with statement to ensure the file is properly closed.

with open("example.txt", "r") as f:
    content = f.read()

Open Modes#

Mode	Description
`"r"`	Read (default). File must exist.
`"w"`	Write. Creates file or overwrites existing content.
`"a"`	Append. Creates file or adds to end of existing content.
`"r+"`	Read and write. File must exist.
`"x"`	Exclusive creation. Fails if file already exists.

Warning

Write Mode Overwrites Opening a file with "w" will erase all existing content. Use "a" to append instead.

File Methods#

Method	Description
`f.read()`	Read entire file as a single string
`f.read(n)`	Read `n` characters
`f.readline()`	Read one line (including `\n`)
`f.readlines()`	Read all lines into a list
`f.write(s)`	Write string `s` to file
`f.writelines(lst)`	Write a list of strings to file
`f.close()`	Close the file
`f.name`	Name of the file

Reading Examples#

# Read entire file
with open("data.txt", "r") as f:
    content = f.read()
    print(content)

# Read line by line
with open("data.txt", "r") as f:
    line = f.readline()
    while line:
        print(line.strip())
        line = f.readline()

# Read all lines into a list
with open("data.txt", "r") as f:
    lines = f.readlines()
    for line in lines:
        print(line.strip())

Writing Examples#

# Write to a file
with open("output.txt", "w") as f:
    f.write("First line\n")
    f.write("Second line\n")

# Append to a file
with open("output.txt", "a") as f:
    f.write("Appended line\n")

Escape Codes#

Code	Description
`\n`	Newline
`\t`	Tab
`\b`	Backspace
`\r`	Carriage return
`\\`	Literal backslash

with open("formatted.txt", "w") as f:
    f.write("Name\tAge\tCity\n")
    f.write("Alice\t30\tLondon\n")
    f.write("Bob\t25\tParis\n")

Practical Examples#

Find and Replace in a File#

with open("data.txt", "r") as f:
    content = f.read()

content = content.replace("old_text", "new_text")

with open("data.txt", "w") as f:
    f.write(content)

Create a List of Files in a Directory#

import os

files = os.listdir("my_folder")
txt_files = [f for f in files if f.endswith(".txt")]

with open("file_list.txt", "w") as out:
    for filename in txt_files:
        out.write(filename + "\n")

Iterating Over Text Files#

Using a for Loop#

The simplest way to iterate line by line:

with open("data.txt", "r") as f:
    for line in f:
        print(line.strip())

Using the fileinput Module#

The fileinput module allows you to iterate over lines from multiple files:

import fileinput

for line in fileinput.input(files=["file1.txt", "file2.txt"]):
    print(f"{fileinput.filename()} line {fileinput.lineno()}: {line.strip()}")

Function	Description
`fileinput.filename()`	Name of the current file being read
`fileinput.lineno()`	Cumulative line number across all files
`fileinput.filelineno()`	Line number within the current file
`fileinput.isfirstline()`	`True` if this is the first line of the current file
`fileinput.close()`	Close the sequence

import fileinput

for line in fileinput.input(files=["data1.txt", "data2.txt"]):
    if fileinput.isfirstline():
        print(f"\n--- Reading: {fileinput.filename()} ---")
    print(line.strip())