def：定义函数
#!/usr/bin/python3
# ----- function without arguments -----
def greeting():
print("-----------------------------")
print(" Hello World ")
print("-----------------------------")
greeting()
# ----- 带参数的函数 -----
def sum_two_numbers(num1, num2):
total = num1 + num2
print("{} + {} = {}".format(num1, num2, total))
sum_two_numbers(3, 4)
# ----- 带返回值的函数 -----
def num_square(num):
return num * num
my_num = 3
print(num_square(2))
print(num_square(my_num))
def关键字用于定义函数
函数必须在使用前定义
一个常见的错误是忘记了def声明语句后的:
函数、控制结构等等代码块都是根据缩进进行区分
默认return值是None
format包含在下一个主题中
$ ./functions.py
-----------------------------
Hello World
-----------------------------
3 + 4 = 7
4
9
默认参数
#!/usr/bin/python3
# ----- function with default valued argument -----
def greeting(style_char='-'):
print(style_char * 29)
print(" Hello World ")
print(style_char * 29)
print("Default style")
greeting()
print("\nStyle character *")
greeting('*')
print("\nStyle character =")
greeting(style_char='=')
通常，如果函数需要根据相关参数改变，可以设定一个默认的行为
$ ./functions_default_arg_value.py
Default style
-----------------------------
Hello World
-----------------------------
Style character *
*****************************
Hello World
*****************************
Style character =
=============================
Hello World
=============================
三引号注释常用于描述函数的功能目的
为了避免示例代码分散注意力，这个指南中不会经常为函数和程序使用文档字符串(docstrings)
查看Docstrings章节的例子和讨论
def num_square(num):
"""
returns square of number
"""
return num * num
进一步阅读
有许多调用函数和其他声明类型的方式，参考下面的链接获取更多信息：
print函数
默认print函数会添加换行符
这可以通过end参数传入我们自己想要的字符串进行更改
>>> print("hi")
hi
>>> print("hi", end='')
hi>>>
>>> print("hi", end=' !!\n')
hi !!
>>>
help函数可以用来从解释器获取函数的快速使用帮助
按q从帮助页面退出
>>> help(print)
Help on built-in function print in module builtins:
print(...)
print(value, ..., sep=' ', end='\n', file=sys.stdout, flush=False)
Prints the values to a stream, or to sys.stdout by default.
Optional keyword arguments:
file: a file-like object (stream); defaults to the current sys.stdout.
sep: string inserted between values, default a space.
end: string appended after the last value, default a newline.
flush: whether to forcibly flush the stream.
传入print函数的多个参数可以通过,分隔
默认的sep值是单个空格
>>> a = 5
>>> b = 2
>>> print(a+b, a-b)
7 3
>>> print(a+b, a-b, sep=' : ')
7 : 3
>>> print(a+b, a-b, sep='\n')
7
3
当打印变量时会调用str方法输出字符结果
所以，除非需要粘连字符串，我们不需要显式地指定输出类型
>>> greeting = 'Hello World'
>>> print(greeting)
Hello World
>>> num = 42
>>> print(num)
42
>>> print(greeting + '. We are learning Python')
Hello World. We are learning Python
>>> print(greeting, '. We are learning Python', sep='')
Hello World. We are learning Python
>>> print("She bought " + num + " apples")
Traceback (most recent call last):
File "", line 1, in
TypeError: Can't convert 'int' object to str implicitly
>>> print("She bought " + str(num) + " apples")
She bought 42 apples
作为备用选项，使用多个参数并根据情况改变sep分隔符
>>> print("She bought", num, "apples")
She bought 42 apples
>>> items = 15
>>> print("No. of items:", items)
No. of items: 15
>>> print("No. of items:", items, sep='')
No. of items:15
为了重定向打印输出到stderr，我们更改file参数
>>> import sys
>>> print("Error!! Not a valid input", file=sys.stderr)
Error!! Not a valid input
str.format()可以用来设定字符串的风格，处理多个字符串时比粘连方式更优雅
>>> num1 = 42
>>> num2 = 7
>>> '{} + {} = {}'.format(num1, num2, num1 + num2)
'42 + 7 = 49'
# 或者将格式保存为一个变量然后在需要的地方使用
>>> op_fmt = '{} + {} = {}'
>>> op_fmt.format(num1, num2, num1 + num2)
'42 + 7 = 49'
>>> op_fmt.format(num1, 29, num1 + 29)
'42 + 29 = 71'
# 在print函数内部使用也当然没问题
>>> print('{} + {} = {}'.format(num1, num2, num1 + num2))
42 + 7 = 49
使用有序的参数
>>> num1
42
>>> num2
7
>>> print("{0} + {1} * {0} = {2}".format(num1, num2, num1 + num2 * num1))
42 + 7 * 42 = 336
数值格式——使用可选的参数值接:和格式风格进行指定
>>> appx_pi = 22 / 7
>>> appx_pi
3.142857142857143
# 在小数点后限制数字的位数
# 数值会进行取舍
>>> print("{0:.2f}".format(appx_pi))
3.14
>>> print("{0:.3f}".format(appx_pi))
3.143
# 对齐
>>> print("{0:<10.3f} and 5.12".format(appx_pi))
3.143 and 5.12
>>> print("{0:>10.3f} and 5.12".format(appx_pi))
3.143 and 5.12
# 用0填充
>>> print("{0:08.3f}".format(appx_pi))
0003.143
不同的基数
>>> print("42 in binary = {:b}".format(42))
42 in binary = 101010
>>> print("42 in octal = {:o}".format(42))
42 in octal = 52
>>> print("241 in hex = {:x}".format(241))
241 in hex = f1
# 通过#添加0b/0o/0x前缀
>>> print("42 in binary = {:#b}".format(42))
42 in binary = 0b101010
>>> hex_str = "{:x}".format(42)
>>> hex_str
'2a'
# 也可以使用内置的format函数
>>> format(42, 'x')
'2a'
>>> format(42, '#x')
'0x2a'
# 将字符串转换为整型
>>> int(hex_str, base=16)
42
>>> int('0x2a', base=16)
42
跟r前缀相似，使用f前缀可以用来表示格式化的字符串
Python v3.6引进
跟str.format()相似，在{}中指定变量/表达式
>>> num1 = 42
>>> num2 = 7
>>> f'{num1} + {num2} = {num1 + num2}'
'42 + 7 = 49'
>>> print(f'{num1} + {num2} = {num1 + num2}')
42 + 7 = 49
>>> appx_pi = 22 / 7
>>> f'{appx_pi:08.3f}'
'0003.143'
>>> f'{20:x}'
'14'
>>> f'{20:#x}'
'0x14'
进一步阅读
range函数
默认参数start=0和step=1,因此它们可以跳过或者根据需要合适地设定
range(stop)
range(start, stop)
range(start, stop, step)
注意range的输出不包含stop值(左闭右开)
查阅列表章节获取列表的讨论和例子
>>> range(5)
range(0, 5)
>>> list(range(5))
[0, 1, 2, 3, 4]
>>> list(range(-2, 2))
[-2, -1, 0, 1]
>>> list(range(1, 15, 2))
[1, 3, 5, 7, 9, 11, 13]
>>> list(range(10, -5, -2))
[10, 8, 6, 4, 2, 0, -2, -4]
type函数
用于检查变量和数值的数据类型
>>> type(5)
>>> type('Hi there!')
>>> type(range(7))
>>> type(None)
>>> type(True)
>>> arr = list(range(4))
>>> arr
[0, 1, 2, 3]
>>> type(arr)
变量作用域
#!/usr/bin/python3
def print_num():
print("Yeehaw! num is visible in this scope, its value is: " + str(num))
num = 25
print_num()
在函数调用之前定义的变量在函数作用域内可视
$ ./variable_scope_1.py
Yeehaw! num is visible in this scope, its value is: 25
在函数代码块内声明的一个变量在代码块之外使用会发生什么？
#!/usr/bin/python3
def square_of_num(num):
sqr_num = num * num
square_of_num(5)
print("5 * 5 = {}".format(sqr_num))
这里，sqr_num声明在square_of_num函数内，在代码块外不能够使用
$ ./variable_scope_2.py
Traceback (most recent call last):
File "./variable_scope_2.py", line 7, in
print("5 * 5 = {}".format(sqr_num))
NameError: name 'sqr_num' is not defined
一种解决办法是使用global关键字
#!/usr/bin/python3
def square_of_num(num):
global sqr_num
sqr_num = num * num
square_of_num(5)
print("5 * 5 = {}".format(sqr_num))
现在，即使在函数外我们也能够使用sqr_num
$ ./variable_scope_3.py
5 * 5 = 25
如果一个变量名在函数内外都进行了定义，函数内的变量不会影响函数外变量的使用
#!/usr/bin/python3
sqr_num = 4
def square_of_num(num):
sqr_num = num * num
print("5 * 5 = {}".format(sqr_num))
square_of_num(5)
print("Whoops! sqr_num is still {}!".format(sqr_num))
注意使用global sqr_num会影响函数外的sqr_num
$ ./variable_scope_4.py
5 * 5 = 25
Whoops! sqr_num is still 4!
进一步阅读
