我试图在Python 3中构建这个bytes对象:
b'3\r\n'
所以我尝试了显而易见的(对我来说)，发现了一个奇怪的行为:
>>> bytes(3) + b'\r\n'
b'\x00\x00\x00\r\n'
显然:
>>> bytes(10)
b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
在阅读文档时，我无法看到字节转换为什么以这种方式工作的任何指针。然而，我确实在这个Python问题中发现了一些关于向字节添加格式的令人惊讶的消息(另见Python 3字节格式化):
http://bugs.python.org/issue3982
这与bytes(int)返回零这样的奇怪情况的交互更加糟糕
and:
如果字节(int)返回该int的ASCIIfication，对我来说会更方便;但说实话，即使是一个错误也比这种行为要好。(如果我想要这种行为——我从来没有——我宁愿它是一个类方法，像“bytes.zero (n)”那样调用。)
谁能给我解释一下这种行为是怎么来的?
当前回答
我对范围为[0,255]的单个int的各种方法的性能很好奇，所以我决定做一些定时测试。
根据下面的时间，以及我从尝试许多不同的值和配置中观察到的总体趋势，struct。Pack似乎是最快的，其次是int。To_bytes、bytes和str.encode(不出意外)是最慢的。注意，结果显示了比所表示的更多的变化，并且int。To_bytes和bytes在测试过程中有时会切换速度排名，但是struct。Pack显然是最快的。
在Windows上的CPython 3.7中的结果:
Testing with 63:
bytes_: 100000 loops, best of 5: 3.3 usec per loop
to_bytes: 100000 loops, best of 5: 2.72 usec per loop
struct_pack: 100000 loops, best of 5: 2.32 usec per loop
chr_encode: 50000 loops, best of 5: 3.66 usec per loop
测试模块(命名为int_to_byte.py):
"""Functions for converting a single int to a bytes object with that int's value."""
import random
import shlex
import struct
import timeit
def bytes_(i):
"""From Tim Pietzcker's answer:
https://stackoverflow.com/a/21017834/8117067
"""
return bytes([i])
def to_bytes(i):
"""From brunsgaard's answer:
https://stackoverflow.com/a/30375198/8117067
"""
return i.to_bytes(1, byteorder='big')
def struct_pack(i):
"""From Andy Hayden's answer:
https://stackoverflow.com/a/26920966/8117067
"""
return struct.pack('B', i)
# Originally, jfs's answer was considered for testing,
# but the result is not identical to the other methods
# https://stackoverflow.com/a/31761722/8117067
def chr_encode(i):
"""Another method, from Quuxplusone's answer here:
https://codereview.stackexchange.com/a/210789/140921
Similar to g10guang's answer:
https://stackoverflow.com/a/51558790/8117067
"""
return chr(i).encode('latin1')
converters = [bytes_, to_bytes, struct_pack, chr_encode]
def one_byte_equality_test():
"""Test that results are identical for ints in the range [0, 255]."""
for i in range(256):
results = [c(i) for c in converters]
# Test that all results are equal
start = results[0]
if any(start != b for b in results):
raise ValueError(results)
def timing_tests(value=None):
"""Test each of the functions with a random int."""
if value is None:
# random.randint takes more time than int to byte conversion
# so it can't be a part of the timeit call
value = random.randint(0, 255)
print(f'Testing with {value}:')
for c in converters:
print(f'{c.__name__}: ', end='')
# Uses technique borrowed from https://stackoverflow.com/q/19062202/8117067
timeit.main(args=shlex.split(
f"-s 'from int_to_byte import {c.__name__}; value = {value}' " +
f"'{c.__name__}(value)'"
))
其他回答
我认为你可以先将int转换为str，然后再转换为byte。 这应该会产生您想要的格式。
bytes(str(your_number),'UTF-8') + b'\r\n'
它在py3.8中为我工作。
这就是它的设计方式——这是有意义的，因为通常，你会在一个可迭代对象上调用bytes，而不是单个整数:
>>> bytes([3])
b'\x03'
文档声明了这一点，以及字节的文档字符串:
>>> help(bytes)
...
bytes(int) -> bytes object of size given by the parameter initialized with null bytes
从python 3.2开始，你可以使用to_bytes:
>>> (1024).to_bytes(2, byteorder='big')
b'\x04\x00'
def int_to_bytes(x: int) -> bytes:
return x.to_bytes((x.bit_length() + 7) // 8, 'big')
def int_from_bytes(xbytes: bytes) -> int:
return int.from_bytes(xbytes, 'big')
因此，x == int_from_bytes(int_to_bytes(x))。 注意，上述编码仅适用于无符号(非负)整数。
对于有符号整数，比特长度的计算有点棘手:
def int_to_bytes(number: int) -> bytes:
return number.to_bytes(length=(8 + (number + (number < 0)).bit_length()) // 8, byteorder='big', signed=True)
def int_from_bytes(binary_data: bytes) -> Optional[int]:
return int.from_bytes(binary_data, byteorder='big', signed=True)
来自bytes docs:
因此，构造函数参数被解释为bytearray()。
然后，从bytearray docs:
可选的source参数可以用几种不同的方式初始化数组: 如果它是一个整数，数组将具有该大小，并将初始化为空字节。
注意，它与2不同。X(其中X >= 6)行为，其中bytes是简单的str:
>>> bytes is str
True
PEP 3112:
2.6的str与3.0的bytes类型有很多不同之处;最值得注意的是，构造函数完全不同。
我对范围为[0,255]的单个int的各种方法的性能很好奇，所以我决定做一些定时测试。
根据下面的时间，以及我从尝试许多不同的值和配置中观察到的总体趋势，struct。Pack似乎是最快的，其次是int。To_bytes、bytes和str.encode(不出意外)是最慢的。注意，结果显示了比所表示的更多的变化，并且int。To_bytes和bytes在测试过程中有时会切换速度排名，但是struct。Pack显然是最快的。
在Windows上的CPython 3.7中的结果:
Testing with 63:
bytes_: 100000 loops, best of 5: 3.3 usec per loop
to_bytes: 100000 loops, best of 5: 2.72 usec per loop
struct_pack: 100000 loops, best of 5: 2.32 usec per loop
chr_encode: 50000 loops, best of 5: 3.66 usec per loop
测试模块(命名为int_to_byte.py):
"""Functions for converting a single int to a bytes object with that int's value."""
import random
import shlex
import struct
import timeit
def bytes_(i):
"""From Tim Pietzcker's answer:
https://stackoverflow.com/a/21017834/8117067
"""
return bytes([i])
def to_bytes(i):
"""From brunsgaard's answer:
https://stackoverflow.com/a/30375198/8117067
"""
return i.to_bytes(1, byteorder='big')
def struct_pack(i):
"""From Andy Hayden's answer:
https://stackoverflow.com/a/26920966/8117067
"""
return struct.pack('B', i)
# Originally, jfs's answer was considered for testing,
# but the result is not identical to the other methods
# https://stackoverflow.com/a/31761722/8117067
def chr_encode(i):
"""Another method, from Quuxplusone's answer here:
https://codereview.stackexchange.com/a/210789/140921
Similar to g10guang's answer:
https://stackoverflow.com/a/51558790/8117067
"""
return chr(i).encode('latin1')
converters = [bytes_, to_bytes, struct_pack, chr_encode]
def one_byte_equality_test():
"""Test that results are identical for ints in the range [0, 255]."""
for i in range(256):
results = [c(i) for c in converters]
# Test that all results are equal
start = results[0]
if any(start != b for b in results):
raise ValueError(results)
def timing_tests(value=None):
"""Test each of the functions with a random int."""
if value is None:
# random.randint takes more time than int to byte conversion
# so it can't be a part of the timeit call
value = random.randint(0, 255)
print(f'Testing with {value}:')
for c in converters:
print(f'{c.__name__}: ', end='')
# Uses technique borrowed from https://stackoverflow.com/q/19062202/8117067
timeit.main(args=shlex.split(
f"-s 'from int_to_byte import {c.__name__}; value = {value}' " +
f"'{c.__name__}(value)'"
))
