Writing Files

Note

Source: Adapted from the C# edition (files/filewrite.rst). Python’s open(path, "w") replaces C#’s StreamWriter. The with statement handles closing automatically, eliminating the common C# bug of forgetting to call Close().

To write a file, pass "w" (write) or "a" (append) as the second argument to open().

Write Mode

Opening a file in write mode creates it if it does not exist, or silently overwrites it if it does:

with open("sample.txt", "w") as f:
    f.write("Hello, file!\n")
    f.write("Second line.\n")

After the with block exits, Python flushes and closes the file. There is no need to call close() explicitly — this is the main advantage over forgetting it in C#.

You can also use print() with the file= keyword argument, which automatically adds a newline:

with open("sample.txt", "w") as f:
    print("Hello, file!", file=f)
    print("Second line.", file=f)

Both approaches produce identical output.

Append Mode

Opening with "a" leaves existing content intact and adds new lines at the end:

with open("log.txt", "a") as f:
    print("New entry added.", file=f)

Example: Copy a File to Upper Case

This example reads an input file line by line and writes each line in upper case to a new output file — a Python translation of the C# copy_upper example:

def copy_upper(src, dst):
    with open(src) as reader:
        with open(dst, "w") as writer:
            for line in reader:
                writer.write(line.upper())

copy_upper("text.txt", "upper_text.txt")

Nested with statements are valid, though Python also allows combining them on one line:

with open(src) as reader, open(dst, "w") as writer:
    for line in reader:
        writer.write(line.upper())

Writing Formatted Data

Use f-strings inside f.write() or print(..., file=f) to produce formatted output:

import math

with open("roots.txt", "w") as f:
    print(f"{'n':>4}  {'sqrt(n)':>10}", file=f)
    for n in range(1, 11):
        print(f"{n:4d}  {math.sqrt(n):10.4f}", file=f)

This writes a neatly aligned table to the file, using the same f-string formatting as on-screen output.