String vs. Bytes

Text in Python 3 is always Unicode and is represented by the str type, and binary data is represented by the bytes type. They cannot be mixed.

Strings can be encoded to bytes, and bytes can be decoded back to strings.



In [1]:

    
s = 'Hello world!'
print(s)
print("length is", len(s))









    



Hello world!
length is 12



In [2]:

    
us = 'Hello 世界!'
print(us)
print("length is", len(us))









    



Hello 世界!
length is 9

Now encode both strings to bytes.



In [3]:

    
bs = s.encode('utf-8')
print(bs)
print("length is", len(bs))









    



b'Hello world!'
length is 12



In [4]:

    
bus = us.encode('utf-8')
print(bus)
print("length is", len(bus))









    



b'Hello \xe4\xb8\x96\xe7\x95\x8c!'
length is 13

Decode back to strings.



In [5]:

    
print(bs.decode('utf-8'))
print(bus.decode('utf-8'))









    



Hello world!
Hello 世界!

Big Endian vs Little Endian



In [6]:

    
num = 258
print(num.to_bytes(2, "big"))
print(num.to_bytes(2, "little"))

print(num.to_bytes(4, "big"))
print(num.to_bytes(4, "little"))









    



b'\x01\x02'
b'\x02\x01'
b'\x00\x00\x01\x02'
b'\x02\x01\x00\x00'

struct package

This module performs conversions between Python values and C structs represented as Python bytes objects.



In [7]:

    
import struct

struct.pack(fmt, v1, v2, …)

Return a bytes object containing the values v1, v2, … packed according to the format string fmt. The arguments must match the values required by the format exactly.

"!" means network endianess (big endian)
"<" means little endian
">" means big endian
"=" means native
"h" measn short integer (2 bytes)



In [8]:

    
x = 256

print("Network endianess")
print(struct.pack('!h', x))

print("Little endian")
print(struct.pack('<h', x))

print("Big endian")
print(struct.pack('>h', x))

print("Native endianess")
print(struct.pack('=h', x))









    



Network endianess
b'\x01\x00'
Little endian
b'\x00\x01'
Big endian
b'\x01\x00'
Native endianess
b'\x00\x01'

struct.unpack(fmt, buffer)

Unpack from the buffer buffer (presumably packed by pack(fmt, ...)) according to the format string fmt. The result is a tuple even if it contains exactly one item. The buffer’s size in bytes must match the size required by the format,



In [9]:

    
bx = struct.pack('!h', x)

print(struct.unpack('!h', bx))
print(struct.unpack('<h', bx))









    



(256,)
(1,)



In [10]:

    
print(struct.unpack('!h', bx)[0])
print(struct.unpack('<h', bx)[0])