>>> num = 5
>>> num > 2
True
>>> num > 3 and num <= 5
True
>>> 3 < num <= 5
True
>>> num % 3 == 0 or num % 5 == 0
True
>>> fav_fiction = 'Harry Potter'
>>> fav_detective = 'Sherlock Holmes'
>>> fav_fiction == fav_detective
False
>>> fav_fiction == "Harry Potter"
True
>>> bool(num)
True
>>> bool(fav_detective)
True
>>> bool(3)
True
>>> bool(0)
False
>>> bool("")
False
>>> bool(None)
False
>>> if -1:
... print("-1 evaluates to True in condition checking")
...
-1 evaluates to True in condition checking
in
操作符的使用对比这种检查方式
>>> def num_chk(n):
... if n == 10 or n == 21 or n == 33:
... print("Number passes condition")
... else:
... print("Number fails condition")
...
>>> num_chk(10)
Number passes condition
>>> num_chk(12)
Number fails condition
和另一种
>>> def num_chk(n):
... if n in (10, 21, 33):
... print("Number passes condition")
... else:
... print("Number fails condition")
...
>>> num_chk(12)
Number fails condition
>>> num_chk(10)
Number passes condition
(10, 21, 33)
是一个元组数据类型,会在后面的章节讲解#!/usr/bin/python3
num = 45
# 单个if
if num > 25:
print("Hurray! {} is greater than 25".format(num))
# if-else
if num % 2 == 0:
print("{} is an even number".format(num))
else:
print("{} is an odd number".format(num))
# if-elif-else
# 可以使用任意数目的elif
if num < 0:
print("{} is a negative number".format(num))
elif num > 0:
print("{} is a positive number".format(num))
else:
print("{} is neither postive nor a negative number".format(num))
:
()
是可选的$ ./if_elif_else.py
Hurray! 45 is greater than 25
45 is an odd number
45 is a positive number
if-else作为条件操作符
#!/usr/bin/python3
num = 42
num_type = 'even' if num % 2 == 0 else 'odd'
print("{} is an {} number".format(num, num_type))
?:
条件操作符if-else
的使用是一种变通方法$ ./if_else_oneliner.py
42 is an even number
#!/usr/bin/python3
number = 9
for i in range(1, 5):
mul_table = number * i
print("{} * {} = {}".format(number, i, mul_table))
range
函数实现start=0
、step=1
,不含stop
值$ ./for_loop.py
9 * 1 = 9
9 * 2 = 18
9 * 3 = 27
9 * 4 = 36
#!/usr/bin/python3
# 持续地询问直到用户输入一个正整数
usr_string = 'not a number'
while not usr_string.isnumeric():
usr_string = input("Enter a positive integer: ")
$ ./while_loop.py
Enter a positive integer: abc
Enter a positive integer: 1.2
Enter a positive integer: 23
$
continue
和break
关键字用于在某些条件下改变正常的循环操作
continue - 跳过循环代码块余下的语句并进入下一次迭代
#!/usr/bin/python3
prev_num = 0
curr_num = 0
print("The first ten numbers in fibonacci sequence: ", end='')
for num in range(10):
print(curr_num, end=' ')
if num == 0:
curr_num = 1
continue
temp = curr_num
curr_num = curr_num + prev_num
prev_num = temp
print("")
continue
放置在循环代码块中的任意位置而不用担心复杂的代码流continue
的使用,查看这里[6]获取更加Python化的操作方式$ ./loop_with_continue.py
The first ten numbers in fibonacci sequence: 0 1 1 2 3 5 8 13 21 34
break - 跳过循环代码块余下的语句(如果有)并进入退出循环代码块
#!/usr/bin/python3
import random
while True:
# 使用range函数注意500没有包含在内
random_int = random.randrange(500)
if random_int % 4 == 0 and random_int % 6 == 0:
break
print("Random number divisible by 4 and 6: {}".format(random_int))
while True:
是常用作无限循环start, stop, step
参数$ ./loop_with_break.py
Random number divisible by 4 and 6: 168
$ ./loop_with_break.py
Random number divisible by 4 and 6: 216
$ ./loop_with_break.py
Random number divisible by 4 and 6: 24
这个while_loop.py例子可以用break
语句重写
>>> while True:
usr_string = input("Enter a positive integer: ")
if usr_string.isnumeric():
break
Enter a positive integer: a
Enter a positive integer: 3.14
Enter a positive integer: 1
>>>
continue
和break
仅影响中间一层对应的循环[1]Python文档 - 真值检验: https://docs.python.org/3/library/stdtypes.html#truth
[2]Python文档 - 编码风格: https://docs.python.org/3/tutorial/controlflow.html#intermezzo-coding-style
[3]模拟三元操作符的更多方法: http://stackoverflow.com/questions/394809/does-python-have-a-ternary-conditional-operator
[4]Python文档 - 迭代工具: https://docs.python.org/3/library/itertools.html
[5]Python docs - 字符串方法: https://docs.python.org/3/library/stdtypes.html#string-methods
[6]这里: https://docs.python.org/3/tutorial/controlflow.html#defining-functions
[7]range: ./Functions.md#range-function
[8]Python文档 - random: https://docs.python.org/3/library/random.html
[9]Python文档 - 循环中的else从句: https://docs.python.org/3/tutorial/controlflow.html#break-and-continue-statements-and-else-clauses-on-loops