Base64 Encoding and Decoding in Python

March 15, 2026 · 3 min read · Updated March 15, 2026 · beginner

python stdlib encoding base64 data

Base64 is a way to represent binary data as ASCII text. It is commonly used when you need to embed binary data in text-based formats like JSON, XML, email attachments, or HTTP headers. Python’s base64 module makes this straightforward.

What is Base64?

Base64 converts binary data into a set of 64 ASCII characters (A-Z, a-z, 0-9, +, and /). Every 3 bytes of binary data become 4 Base64 characters. This makes the encoded data about 33% larger than the original, but it is safe for text-only contexts.

You will see Base64 used for:

Embedding images in CSS or HTML
Transmitting binary data in JSON APIs
Encoding credentials in HTTP headers
Storing binary files in text-based formats

Basic Encoding

The simplest way to encode data is with b64encode():

import base64

# Encode a string (must convert to bytes first)
message = "Hello, World!"
encoded = base64.b64encode(message.encode("utf-8"))
print(encoded)  # b"SGVsbG8sIFdvcmxkIQ=="

The result is bytes, not a string. If you need a string:

import base64

message = "Python is awesome"
encoded = base64.b64encode(message.encode("utf-8"))
encoded_str = encoded.decode("utf-8")
print(encoded_str)  # UHl0aG9uIGlzIGF3ZXNvbWU=

Basic Decoding

To decode Base64 back to bytes, use b64decode():

import base64

encoded_str = "SGVsbG8sIFdvcmxkIQ=="
decoded_bytes = base64.b64decode(encoded_str)
print(decoded_bytes)  # b"Hello, World!"

# Convert bytes back to string
decoded_str = decoded_bytes.decode("utf-8")
print(decoded_str)  # Hello, World!

Encoding Files

You can encode entire files, which is useful for embedding small images or sending binary data over text-only channels:

import base64

# Read and encode an image file
with open("image.png", "rb") as f:
    image_data = f.read()
    encoded = base64.b64encode(image_data)
    
print(f"Original size: {len(image_data)} bytes")
print(f"Encoded size: {len(encoded)} bytes")
# output: Original size: 1024 bytes
# output: Encoded size: 1376 bytes

Decoding Files

Similarly, you can decode Base64 data back to a file:

import base64

# Decode and save to file
encoded_data = "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNk+M9QDwADhgGAWjR9awAAAABJRU5ErkJggg=="

with open("output.png", "wb") as f:
    f.write(base64.b64decode(encoded_data))
    
print("File written successfully")

URL-Safe Base64

The standard Base64 uses + and /, which can cause problems in URLs. Use URL-safe encoding instead:

import base64

# Standard Base64 (can have + and /)
standard = base64.b64encode(b"hello+world/")
print(standard)  # b"aGVsbG8rd29ybGQv"

# URL-safe Base64 (uses - and _ instead)
url_safe = base64.urlsafe_b64encode(b"hello+world/")
print(url_safe)  # b"aGVsbG8rd29ybGQv"

Working with Padding

Base64 encoding always produces output whose length is a multiple of 4. The = characters are padding:

import base64

# Short strings need padding
print(base64.b64encode(b"a"))    # b"YQ=="  (2 padding chars)
print(base64.b64encode(b"ab"))   # b"YWI="  (1 padding char)
print(base64.b64encode(b"abc"))  # b"YWJj"  (no padding)
print(base64.b64encode(b"abcd"))  # b"YWJjZA=="  (2 padding chars)

# You can strip padding if you handle it during decoding
def encode_no_padding(data):
    return base64.b64encode(data).decode("ascii").rstrip("=")

def decode_no_padding(data):
    # Add padding back before decoding
    padding = 4 - (len(data) % 4)
    if padding != 4:
        data += "=" * padding
    return base64.b64decode(data)

# Example
encoded = encode_no_padding(b"hello")
print(encoded)  # aGVsbG8
decoded = decode_no_padding(encoded)
print(decoded)  # b"hello"

Common Use Cases

Sending Binary Data in JSON

import base64
import json

# Imagine you have an image to send as JSON
image_data = b"\x89PNG\r\n\x1a\n..."  # Actual PNG header

payload = {
    "filename": "photo.png",
    "data": base64.b64encode(image_data).decode("utf-8"),
    "mime_type": "image/png"
}

json_str = json.dumps(payload)
print(json_str)
# output: {"filename": "photo.png", "data": "iVBORw0KGgo...", "mime_type": "image/png"}

Basic Authentication Header

import base64

username = "admin"
password = "secret123"

# Create Basic Auth header value
credentials = f"{username}:{password}"
encoded = base64.b64encode(credentials.encode("utf-8"))
auth_header = f"Basic {encoded.decode('utf-8')}"

print(auth_header)
# output: Basic YWRtaW46c2VjcmV0MTIz

Encoding API Responses

import base64
import json

# Some APIs return binary data Base64-encoded
api_response = '{"data": "SGVsbG8gZnJvbCBBUEk="}'

response = json.loads(api_response)
decoded_message = base64.b64decode(response["data"]).decode("utf-8")
print(decoded_message)  # Hello from API