int(x, [base])
功能：
函数的作用是将一个数字或base类型的字符串转换成整数。（ Python视频教程）
函数原型：
int(x=0)
int(x, base=10)，base缺省值为10，也就是说不指定base的值时，函数将x按十进制处理。
适用Python版本：
Python2.x
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Python3.x
注意：
1. x 可以是数字或字符串，但是base被赋值后 x 只能是字符串
2. x 作为字符串时必须是 base 类型，也就是说 x 变成数字时必须能用 base 进制表示
Python英文文档解释：
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).
The integer type is described in Numeric Types — int, float, complex.
Changed in version 3.4: If base is not an instance of int and the base object has a base.__index__ method, that method is called to obtain an integer for the base. Previous versions used base.__int__ instead of base.__index__.
Changed in version 3.6: Grouping digits with underscores as in code literals is allowed.
代码实例：
1. x 是数字的情况：
int(3.14) # 3 int(2e2) # 200 int(100, 2) # 出错，base 被赋值后函数只接收字符串
2. x 是字符串的情况：
int('23', 16) # 35
int('Pythontab', 8) # 出错，Pythontab不是个8进制数3. base 可取值范围是 2~36，囊括了所有的英文字母(不区分大小写)，十六进制中F表示15，那么G将在二十进制中表示16，依此类推....Z在三十六进制中表示35
int('FZ', 16) # 出错，FZ不能用十六进制表示
int('FZ', 36) # 5754. 字符串 0x 可以出现在十六进制中，视作十六进制的符号，同理 0b 可以出现在二进制中，除此之外视作数字 0 和字母 x
int('0x10', 16) # 16，0x是十六进制的符号
int('0x10', 17) # 出错，'0x10'中的 x 被视作英文字母 x
int('0x10', 36) # 42804，36进制包含字母 x 
