Detailed introduction to Python's built-in int function

英文文档：

class int(x=0) class int(x, base=10)

Return an integer object constructed from a number or string x, or return 0 if no arguments are given. If x is a number, return x.__int__(). For floating point numbers, this truncates towards zero.

If x is not a number or if base is given, then x must be a string, bytes, or bytearray instance representing an integer literal in radix base. Optionally, the literal can be preceded by + or - (with no space in between) and surrounded by whitespace. A base-n literal consists of the digits 0 to n-1, with a to z (or A to Z) having values 10 to 35. The default base is 10. The allowed values are 0 and 2-36. Base-2, -8, and -16 literals can be optionally prefixed with 0b/0B, 0o/0O, or 0x/0X, as with integer literals in code. Base 0 means to interpret exactly as a code literal, so that the actual base is 2, 8, 10, or 16, and so that int('010', 0) is not legal, while int('010') is, as well as int('010', 8).

说明：

　　1. 不传入参数时，得到结果0。

>>> int()
0

　　2. 传入数值时，调用其__int__()方法，浮点数将向下取整。

>>> int(3)3
>>> int(3.6)3

　　3. 传入字符串时，默认以10进制进行转换。

>>> int('36')36
>>> int('3.6')
Traceback (most recent call last):
  File "<pyshell#2>", line 1, in <module>
    int(&#39;3.6&#39;)
ValueError: invalid literal for int() with base 10: &#39;3.6&#39;

　　4. 字符串中允许包含"+"、"-"号，但是加减号与数值间不能有空格，数值后、符号前可出现空格。

>>> int('+36')36
>>> int('-36')-36
>>> int('   -36        ')-36
>>> int(' - 36        ')
Traceback (most recent call last):
  File "<pyshell#7>", line 1, in <module>
    int(&#39; - 36        &#39;)
ValueError: invalid literal for int() with base 10: &#39; - 36

　　5. 传入字符串，并指定了进制，则按对应进制将字符串转换成10进制整数。

>>> int('01',2)1
>>> int('02',3)2
>>> int('07',8)7
>>> int('0f',16)15

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