Reading Files#

Note

Source: Adapted from the C# edition (files/fileread.rst). Python’s with open(...) as f: replaces C#’s StreamReader. The for line in f: loop replaces the while (!reader.EndOfStream) pattern. f.read() corresponds to C#’s ReadToEnd().

The open() function connects a Python program to a file on disk. The with statement ensures the file is automatically closed when the block exits — even if an error occurs.

Opening and Reading Line by Line#

The most common pattern reads a file one line at a time:

with open("sample.txt") as f:
    for line in f:
        print(line, end="")

Each string yielded by for line in f: includes the trailing newline character, so we pass end="" to print to avoid a double newline. To remove the newline explicitly:

with open("sample.txt") as f:
    for line in f:
        line = line.rstrip()
        print(line)

rstrip() strips trailing whitespace (including \n and \r\n).

You can pass a pathlib.Path object anywhere a filename string is expected:

from pathlib import Path

with open(Path("data") / "numbers.txt") as f:
    for line in f:
        print(line.rstrip())

Reading the Entire File at Once#

Two methods read everything in one call:

f.read() returns the whole file as a single string (with embedded newlines).
f.readlines() returns a list of line strings, each ending with \n.

with open("sample.txt") as f:
    contents = f.read()
print(contents)

with open("sample.txt") as f:
    lines = f.readlines()
print(len(lines), "lines")

Use f.read() when you need the whole text at once (e.g., to pass to another function). Use the for line in f: loop when the file may be large, since it reads one line at a time without loading everything into memory.

Example: Summing a File of Numbers#

Suppose numbers.txt contains one integer per line:

def sum_file(filename):
    total = 0
    with open(filename) as f:
        for line in f:
            total += int(line.strip())
    return total

print(sum_file("numbers.txt"))

Output:

Checking Whether a File Exists#

Before opening a file that might not exist, check with Path.exists():

from pathlib import Path

filename = input("Enter file name: ")
if not Path(filename).exists():
    print("File not found.")
else:
    with open(filename) as f:
        for line in f:
            print(line.rstrip())

This prevents a FileNotFoundError and lets you give a clear message to the user.