Base64 in Python

Python's base64 module (part of the standard library) provides a comprehensive set of functions for Base64 encoding and decoding. Unlike JavaScript, Python makes a clear distinction between byte strings (bytes) and text strings (str), which you must handle correctly.

Basic string encoding and decoding:

import base64

# Encoding: str -> bytes -> base64 bytes -> str
text = "Hello, World!"
encoded = base64.b64encode(text.encode("utf-8")).decode("ascii")
print(encoded)  # "SGVsbG8sIFdvcmxkIQ=="

# Decoding: str -> bytes -> decoded bytes -> str
decoded = base64.b64decode(encoded).decode("utf-8")
print(decoded)  # "Hello, World!"

The key detail is that b64encode() takes bytes and returns bytes. You must .encode("utf-8") your string first, and .decode("ascii") the result if you need a string.

File encoding:

import base64

# Encode a file
with open("image.png", "rb") as f:
    encoded = base64.b64encode(f.read()).decode("ascii")

# Decode back to file
with open("output.png", "wb") as f:
    f.write(base64.b64decode(encoded))

URL-safe Base64:

# Uses - and _ instead of + and /
encoded = base64.urlsafe_b64encode(data).decode("ascii")
decoded = base64.urlsafe_b64decode(encoded)

Other variants available:

• base64.b32encode() / b32decode() -- Base32 encoding
• base64.b16encode() / b16decode() -- Base16 (hexadecimal)
• base64.a85encode() / a85decode() -- Ascii85 encoding
• base64.encodebytes() -- adds newlines every 76 characters (MIME style)

Common mistakes in Python:

• Passing a str directly to b64encode() instead of bytes. This raises a TypeError.
• Using b64encode() when you need MIME-formatted output with line breaks. Use encodebytes() instead.
• Forgetting to open files in binary mode ("rb" / "wb"), which causes encoding errors or file corruption on Windows.
• Not handling padding correctly when decoding URL-safe Base64 from external sources. Python's urlsafe_b64decode() is lenient about missing padding, but adding it explicitly is safer.