The 'b' Prefix in Python String Literals


In Python, you may have come across a string literal prefixed with the 'b' character and wondered what it means. This Byte aims to shed light on this feature of Python strings, its connection with Unicode and binary strings, and how it can be used in your code.

Binary Strings in Python

The 'b' prefix in Python string literals is used to create a bytes literal. When you prefix a string with 'b', you're telling Python to treat the string as binary data. This can be useful when you need to work with data at the byte level.

# Python binary string with 'b' prefix
bs = b'Hello, Python!'


b'Hello, Python!'

In the example above, the 'b' prefix tells Python that 'Hello, Python!' is a bytes literal, not a regular string. When you print it, Python shows the 'b' prefix in the output to indicate that it's a bytes object, not a str object.

One thing to remember is that a bytes object is immutable. This means that once you've defined a bytes object, you can't change its elements. However, you can convert a bytes object to a bytearray object, which is mutable.

# Python bytearray
ba = bytearray(bs)
ba[0] = 74  # Change 'H' (72 in ASCII) to 'J' (74 in ASCII)


bytearray(b'Jello, Python!')

In the example above, we first convert the bytes object to a bytearray object. Then we change the first byte of the bytearray. When we print it, we see that the first character has changed from 'H' to 'J'.

The 'b' Prefix with Different Data Types

Now, let's explore how the 'b' prefix works with different data types in Python. When you put a 'b' in front of a string literal, Python interprets the string as bytes. This is particularly useful when you're dealing with binary data, such as data read from a binary file or received over a network.

Let's take an example. Try running the following code:

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b_string = b'Hello, StackAbuse readers!'

You'll see this output:

b'Hello, StackAbuse readers!'

You can see that the 'b' prefix is preserved in the output, indicating that this is a bytes object, not a regular string.

Note: When you're using the 'b' prefix, you can only include ASCII characters in your string. If you try to include a non-ASCII character, Python will raise a SyntaxError.

For example:

b_string = b'Hello, StackAbuse readers! šŸ‘‹'

This will raise a SyntaxError:

  File "<stdin>", line 1
SyntaxError: bytes can only contain ASCII literal characters.

This is because the 'b' prefix tells Python to interpret the string as bytes, and bytes can only represent ASCII characters.

Why use binary strings?

Let's see some use-cases for opting for byte strings over other data types:

Memory Usage

Byte strings are more memory-efficient when you're dealing with binary data that doesn't require the additional features offered by regular strings. In Python, a regular string is Unicode and therefore can require multiple bytes to represent a single character. On the other hand, a byte string only uses a single byte per element, which can be particularly useful when working with large datasets or when manipulating binary files.

I/O Operations

If you're dealing with file I/O that involves binary files, like images or executables, byte strings are almost a necessity. When you read a binary file, the data is read into memory as bytes. If you were to use regular strings in this scenario, you'd have to perform additional operations to convert the binary data to a Unicode string representation, which could be both time-consuming and error-prone.

Network Programming

In the realm of network programming, byte strings can significantly simplify tasks. Most networking protocols expect data to be sent in a bytes-like format. By using byte strings, you can easily construct packets without having to worry about text encodings and character sets.

Compatibility with C Libraries

Another scenario where byte strings come in handy is when interfacing with C libraries. The C programming language often employs byte arrays for strings. If you're using Python's ctypes library or other similar methods to call C functions, you'll find that byte strings provide a hassle-free way to pass string data between Python and C.

Simplified Binary Operations

Byte strings allow for easier bitwise operations, like bitwise AND, OR, and XOR, which might be cumbersome when working with regular strings. If you're working on encryption algorithms, data compression, or other tasks that require bitwise manipulation, byte strings make these operations much easier.


In this Byte, we've explored the use of the 'b' prefix in Python string literals. We've learned that this prefix tells Python to interpret the string as bytes, rather than a regular string. This is useful when working with binary data, but it also means that you can only include ASCII characters in your string.

Python is a powerful language with a lot of features designed to make your life as a programmer easier. The 'b' prefix is just one of these features, but it's a helpful one to understand if you're working with binary data.

Last Updated: September 5th, 2023
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