英文文档：
class bytearray([source[, encoding[, errors]]])
Return a new array of bytes. The bytearray class is a mutable sequence of integers in the range 0 <= x < 256. It has most of the usual methods of mutable sequences, described in Mutable Sequence Types, as well as most methods that the bytes type has, see Bytes and Bytearray Operations.
The optional source parameter can be used to initialize the array in a few different ways:
- If it is a string, you must also give the encoding (and optionally, errors) parameters; bytearray() then converts the string to bytes using str.encode()
- If it is an integer, the array will have that size and will be initialized with null bytes.
- If it is an object conforming to the buffer interface, a read-only buffer of the object will be used to initialize the bytes array.
- If it is an iterable, it must be an iterable of integers in the range 0 <= x < 256, which are used as the initial contents of the array.
Without an argument, an array of size 0 is created.
说明：
1.返回值为一个新的字节数组
2.当3个参数都不传的时候，返回长度为0的字节数组
>>> b = bytearray()
>>> b
bytearray(b'')
>>> len(b)
0
3.当source参数为字符串时，encoding参数也必须提供，函数将字符串使用str.encode方法转换成字节数组
>>> bytearray('中文')
Traceback (most recent call last):
File "<pyshell#48>", line 1, in <module>
bytearray('中文')
TypeError: string argument without an encoding
>>> bytearray('中文','utf-8')
bytearray(b'\xe4\xb8\xad\xe6\x96\x87')
4.当source参数为整数时，返回这个整数所指定长度的空字节数组
>>> bytearray(2)
bytearray(b'\x00\x00')
>>> bytearray(-2) #整数需大于0，使用来做数组长度的
Traceback (most recent call last):
File "<pyshell#51>", line 1, in <module>
bytearray(-2)
ValueError: negative count
5.当source参数为实现了buffer接口的object对象时，那么将使用只读方式将字节读取到字节数组后返回
6.当source参数是一个可迭代对象，那么这个迭代对象的元素都必须符合0 <= x < 256，以便可以初始化到数组里
>>> bytearray([1,2,3])
bytearray(b'\x01\x02\x03')
>>> bytearray([256,2,3]) #不在0-255范围内报错
Traceback (most recent call last):
File "<pyshell#53>", line 1, in <module>
bytearray([256,2,3])
ValueError: byte must be in range(0, 256)
