if语句 语句如果

>>> x = int(input("Please enter an integer: "))

Please enter an integer: 42

>>> if x < 0:

... x = 0

... print('Negative changed to zero')

... elif x == 0:

... print('Zero')

... elif x == 1:

... print('Single')

... else:

... print('More')

...

More

for语句

>>> # Measure some strings: 测量一些字符串

>>> words = ['cat', 'window', 'defenestrate']

>>> for w in words:

... print(w, len(w))

...

cat 3

window 6

defenestrate 12

range() 功能

>>> for i in range(5):

... print(i)

...

0

1

2

3

4

>>> list(range(5, 10))

[5, 6, 7, 8, 9]

>>> list(range(0, 10, 3))

[0, 3, 6, 9]

>>> list(range(-10, -100, -30))

[-10, -40, -70]

range() 和 len() 组合

>>> a = ['Mary', 'had', 'a', 'little', 'lamb']

>>> for i in range(len(a)):

... print(i, a[i])

...

0 Mary

1 had

2 a

3 little

4 lamb

>>> range(10)

range(0, 10)

sum()

>>> sum(range(4)) # 0 + 1 + 2 + 3

6

break 中断语句

>>> for n in range(2, 10):

... for x in range(2, n):

... if n % x == 0:

... print(n, 'equals', x, '*', n//x)

... break

... else:

... # loop fell through without finding a factor

... print(n, 'is a prime number')

...

2 is a prime number # 2是质数

3 is a prime number

4 equals 2 * 2 # 4等于2 * 2

5 is a prime number

6 equals 2 * 3

7 is a prime number

8 equals 2 * 4

9 equals 3 * 3

continue 继续声明

>>> for num in range(2, 10):

... if num % 2 == 0:

... print("Found an even number", num)

... continue

... print("Found an odd number", num)

...

Found an even number 2

Found an odd number 3

Found an even number 4

Found an odd number 5

Found an even number 6

Found an odd number 7

Found an even number 8

Found an odd number 9

pass

>>> while True:

... pass # Busy-wait for keyboard interrupt (Ctrl+C)

...

>>> class MyEmptyClass:

... pass

...

>>> def initlog(*args):

... pass # Remember to implement this!

...

定义函数

>>> def fib(n): # write Fibonacci series up to n

... """Print a Fibonacci series up to n"""

... a, b = 0, 1

... while a < n:

... print(a, end=' ')

... a, b = b, a+b

... print()

...

>>> # Now call the function we just defined: 现在调用刚刚定义的函数

>>> fib(2000)

0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597

>>> fib

<function fib at 0x000001DB1C30FEE0>

>>> f = fib

>>> f(100)

0 1 1 2 3 5 8 13 21 34 55 89

>>> def fib2(n):

... result = []

... a, b = 0, 1

... while a < n:

... result.append(a)

... a, b = b, a+b

... return result

...

>>> f100 = fib2(100)

>>> f100

[0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]

默认参数值

>>> def ask_ok(prompt, retries=4, reminder='Please try again!'):

... while True:

... ok = input(prompt)

... if ok in ('y', 'ye', 'yes'):

... return True

... if ok in ('n', 'no', 'nop', 'nope'):

... return False

... retries = retries - 1

... if retries < 0:

... raise ValueError('invalid user response')

... print(reminder)

此函数可以通过以下几种方式调用

只给出强制性参数：ask_ok('Do you really want to quit?')

>>> ask_ok('Do you really want to quit?')

Do you really want to quit?yyyyyyyy

Please try again!

Do you really want to quit?ye

True

>>> ask_ok('Do you really want to quit?')

Do you really want to quit?no

False

给出一个可选的参数：ask_ok('OK to overwrite the file?', 2)>>>

>>> ask_ok('OK to overwrite the file?', 3)

OK to overwrite the file?

Please try again!

OK to overwrite the file?

Please try again!

OK to overwrite the file?

Please try again!

OK to overwrite the file?

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

File "<stdin>", line 10, in ask_ok

ValueError: invalid user response

甚至给出所有参数：ask_ok('OK to overwrite the file?', 2, 'Come on, only yes or no!')

>>> ask_ok('OK to overwrite the file?', 2, 'Come on, only yes or no!')

OK to overwrite the file?ttt

Come on, only yes or no!

OK to overwrite the file?ye

True

>>> i = 5

>>> def f(arg=i):

... print(arg)

...

>>> i = 6

>>> f()

5 # 将打印5

>>> def f(a, L=[]):

... L.append(a)

... return L

...

>>> print(f(1))

[1]

>>> print(f(2))

[1, 2]

>>> print(f(3))

[1, 2, 3]

>>> def f(a, L=None):

... if L is None:

... L = []

... L.append(a)

... return L