file.read() reads and returns the entire remaining contents of an open file as a single string (or bytes object, in binary mode), optionally limited to a given number of characters.

Syntax

f.read()
f.read(size)

Deep Dive Course

Called with no argument, read() consumes everything from the file's current position to the end and returns it as one string, which is convenient for small files but can use a lot of memory for large ones, since the whole content is loaded at once. Passing an integer reads at most that many characters, or bytes in binary mode, and advances the file's position accordingly, so calling it repeatedly reads the file in chunks. Once read() reaches the end of the file, subsequent calls return an empty string.

1Understanding read()

💡

For large files, read the file line by line (iterating over it directly, or with readline()) or in fixed-size chunks with read(size), instead of calling plain read() and loading the entire file into memory at once.

editor.html

with open("notes.txt", "w") as f:
    f.write("Line one\nLine two")

with open("notes.txt", "r") as f:
    contents = f.read()
print(contents)

localhost:3000

2Practical Example

Here is a real-world application of read() showing how it is used in production Python code.

editor.html

with open("notes.txt", "r") as f:
    chunk = f.read(4)
    print(repr(chunk))
    print(repr(f.read(4)))

localhost:3000

3Best Practices

Follow these guidelines when working with read():

1. Use read() without arguments only for files you know are reasonably small

2. Read large files in chunks with read(size) or line by line to avoid loading everything into memory at once

3. Always open and read files inside a with block so the file is closed automatically afterward

⚠️

Tip: For large files, read the file line by line (iterating over it directly, or with readline()) or in fixed-size chunks with read(size), instead of calling plain read() and loading the entire file into memory at once.

editor.html

with open("notes.txt", "w") as f:
    f.write("Line one\nLine two")

with open("notes.txt", "r") as f:
    contents = f.read()
print(contents)

localhost:3000

Examples

Example 01Basic Usage

with open("notes.txt", "w") as f:
    f.write("Line one\nLine two")

with open("notes.txt", "r") as f:
    contents = f.read()
print(contents)

Example 02Advanced Example

with open("notes.txt", "r") as f:
    chunk = f.read(4)
    print(repr(chunk))
    print(repr(f.read(4)))

Best Practices

Use read() without arguments only for files you know are reasonably small
Read large files in chunks with read(size) or line by line to avoid loading everything into memory at once
Always open and read files inside a with block so the file is closed automatically afterward

Interview Question

Why is calling read() with no arguments risky for very large files?▼

Hint: Think about where the file's contents end up while the program runs.

read() with no argument loads the entire remaining contents of the file into memory as one single string, or bytes object, before returning anything. For a file much larger than available RAM, like a multi-gigabyte log file, this can exhaust memory or make the program extremely slow. Reading in fixed-size chunks with read(size), or iterating over the file line by line, processes the data incrementally instead, using only a small, bounded amount of memory at any given time.

Exercises

MediumPractice using read() in a real scenario.

View Solution

with open("notes.txt", "w") as f:
    f.write("Line one
Line two")

with open("notes.txt", "r") as f:
    contents = f.read()
print(contents)

Frequently Asked Questions

Why is calling read() with no arguments risky for very large files?▼

Related Functions

readline()with-statementfile-modes