binascii — Convert between binary and ASCII

The binascii module contains a number of methods to convert between binary and various ASCII-encoded binary representations. Normally, you will not use these functions directly but use wrapper modules like base64 instead. The binascii module contains low-level functions written in C for greater speed that are used by the higher-level modules.


a2b_* functions accept Unicode strings containing only ASCII characters. Other functions only accept bytes-like objects (such as bytes, bytearray and other objects that support the buffer protocol).

Changed in version 3.3: ASCII-only unicode strings are now accepted by the a2b_* functions.

The binascii module defines the following functions:


Convert a single line of uuencoded data back to binary and return the binary data. Lines normally contain 45 (binary) bytes, except for the last line. Line data may be followed by whitespace.

binascii.b2a_uu(data, *, backtick=False)

Convert binary data to a line of ASCII characters, the return value is the converted line, including a newline char. The length of data should be at most 45. If backtick is true, zeros are represented by '`' instead of spaces.

Changed in version 3.7: Added the backtick parameter.

binascii.a2b_base64(string, /, *, strict_mode=False)

Convert a block of base64 data back to binary and return the binary data. More than one line may be passed at a time.

If strict_mode is true, only valid base64 data will be converted. Invalid base64 data will raise binascii.Error.

Valid base64:

  • Conforms to RFC 3548.

  • Contains only characters from the base64 alphabet.

  • Contains no excess data after padding (including excess padding, newlines, etc.).

  • Does not start with a padding.

Changed in version 3.11: Added the strict_mode parameter.

binascii.b2a_base64(data, *, newline=True)

Convert binary data to a line of ASCII characters in base64 coding. The return value is the converted line, including a newline char if newline is true. The output of this function conforms to RFC 3548.

Changed in version 3.6: Added the newline parameter.

binascii.a2b_qp(data, header=False)

Convert a block of quoted-printable data back to binary and return the binary data. More than one line may be passed at a time. If the optional argument header is present and true, underscores will be decoded as spaces.

binascii.b2a_qp(data, quotetabs=False, istext=True, header=False)

Convert binary data to a line(s) of ASCII characters in quoted-printable encoding. The return value is the converted line(s). If the optional argument quotetabs is present and true, all tabs and spaces will be encoded. If the optional argument istext is present and true, newlines are not encoded but trailing whitespace will be encoded. If the optional argument header is present and true, spaces will be encoded as underscores per RFC 1522. If the optional argument header is present and false, newline characters will be encoded as well; otherwise linefeed conversion might corrupt the binary data stream.

binascii.crc_hqx(data, value)

Compute a 16-bit CRC value of data, starting with value as the initial CRC, and return the result. This uses the CRC-CCITT polynomial x16 + x12 + x5 + 1, often represented as 0x1021. This CRC is used in the binhex4 format.

binascii.crc32(data[, value])

Compute CRC-32, the unsigned 32-bit checksum of data, starting with an initial CRC of value. The default initial CRC is zero. The algorithm is consistent with the ZIP file checksum. Since the algorithm is designed for use as a checksum algorithm, it is not suitable for use as a general hash algorithm. Use as follows:

print(binascii.crc32(b"hello world"))
# Or, in two pieces:
crc = binascii.crc32(b"hello")
crc = binascii.crc32(b" world", crc)
print('crc32 = {:#010x}'.format(crc))

Changed in version 3.0: The result is always unsigned.

binascii.b2a_hex(data[, sep[, bytes_per_sep=1]])
binascii.hexlify(data[, sep[, bytes_per_sep=1]])

Return the hexadecimal representation of the binary data. Every byte of data is converted into the corresponding 2-digit hex representation. The returned bytes object is therefore twice as long as the length of data.

Similar functionality (but returning a text string) is also conveniently accessible using the bytes.hex() method.

If sep is specified, it must be a single character str or bytes object. It will be inserted in the output after every bytes_per_sep input bytes. Separator placement is counted from the right end of the output by default, if you wish to count from the left, supply a negative bytes_per_sep value.

>>> import binascii
>>> binascii.b2a_hex(b'\xb9\x01\xef')
>>> binascii.hexlify(b'\xb9\x01\xef', '-')
>>> binascii.b2a_hex(b'\xb9\x01\xef', b'_', 2)
>>> binascii.b2a_hex(b'\xb9\x01\xef', b' ', -2)
b'b901 ef'

Changed in version 3.8: The sep and bytes_per_sep parameters were added.


Return the binary data represented by the hexadecimal string hexstr. This function is the inverse of b2a_hex(). hexstr must contain an even number of hexadecimal digits (which can be upper or lower case), otherwise an Error exception is raised.

Similar functionality (accepting only text string arguments, but more liberal towards whitespace) is also accessible using the bytes.fromhex() class method.

exception binascii.Error

Exception raised on errors. These are usually programming errors.

exception binascii.Incomplete

Exception raised on incomplete data. These are usually not programming errors, but may be handled by reading a little more data and trying again.

See also

Module base64

Support for RFC compliant base64-style encoding in base 16, 32, 64, and 85.

Module quopri

Support for quoted-printable encoding used in MIME email messages.