Python 基本语法

• 加减乘除

x = 3
print(type(x))  # <class 'int'>
print(x, x + 1, x - 1, x * 2, x ** 2)  # 3 4 2 6 9

x += 1
print(x)  # 4

x *= 2
print(x)  # 8

y = 2.5
print(type(y))  # <class 'float'>
print(y, y +1, y * 2, y ** 2)  # 2.5 3.5 5.0 6.25

• 布尔

# Booleans
t = True
f = False
print(type(t))  # <class 'bool'>
print(t and f, t or f, not t, t != f, t == f)  # False True False True False

• 字符串

hello = "hello"
world = "world"
hw = hello + world
hw12 = "%s %s %d" % (hello, world, 12)
print(hello, world, "!" ";", hw, ";", hw12)  # hello world !; helloworld ; hello world 12

s = "helLo"
print(s.capitalize())  # Hello
print(s.upper())       # HELLO
print(s.lower())       # hello
print(s.rjust(7))      # __helLo
print(s.ljust(7))      # helLo__
print(s.center(7))     # _helLo_
print(s.replace("l",'(ell)'))  # he(ell)Lo
print('     wo  rld     '.strip())  # wo  rld

• 列表

xs = [3, 1, 2]
print(xs, xs[1], xs[-1])  # [3, 1, 2] 1 2
xs.append('bar')
print(xs)                 # [3, 1, 2, 'bar']
x = xs.pop()
print(x, xs)              # bar [3, 1, 2]

• 切片

nums = list(range(5))
print(nums)       # [0, 1, 2, 3, 4]
print(nums[-1])   # 4
print(nums[2:4])  # [2, 3]
print(nums[2:])   # [2, 3, 4]
print(nums[:2])   # [0, 1]
print(nums[:])    # [0, 1, 2, 3, 4]
print(nums[:-1])  # [0, 1, 2, 3]
nums[0:2] = [8, 9]
print(nums)       # [8, 9, 2, 3, 4]

• 循环访问列表

animals = ['cat', 'dog', 'monkey']
for animal in animals:
    print(animal)

cat
dog
monkey

for idx, animal in enumerate(animals):
    print('%d:%s' % (idx + 1, animal))

1:cat
2:dog
3:monkey

nums = [0, 1, 2, 3, 4]
squares = []
for x in nums:
    squares.append(x ** 2)
print(squares)  # [0, 1, 4, 9, 16]

nums = [0, 1, 2, 3, 4]
squares = [x ** 3 for x in nums]
print(squares)  # [0, 1, 8, 27, 64]

nums = [0, 1, 2, 3, 4]
squares = [x ** 3 for x in nums if x % 2 == 0]
print(squares)  # [0, 8, 64]

• 字典

d = {'cat': 'cute', 'dog': 'furry'}
print(d['cat'])  # cute
print('cat' in d)  # True
d['fish'] = 'wet'
print(d)  # {'fish': 'wet', 'cat': 'cute', 'dog': 'furry'}
print(d.get('fish', 'N/A'))  # wet
print(d.get('fi  sh', 'N/A'))  # N/A
del d['fish']
print(d.get('fish', 'N/A'))  # N/A

• 循环访问字典

d = {'person': 2, 'cat': 4, 'spider': 8}
for animal in d:
    legs = d[animal]
    print('A %s has %d legs' % (animal, legs))

A spider has 8 legs
A cat has 4 legs
A person has 2 legs

for animal, legs in d.items():
    print('A %s has %d legs' % (animal, legs))

A cat has 4 legs
A person has 2 legs
A spider has 8 legs

nums = [0, 1, 2, 3, 4]
even_num_to_square = {x+1: x ** 2 for x in nums if x % 2 ==0}
print(even_num_to_square)  # {1: 0, 3: 4, 5: 16}

• 集合

animals = {'dog', 'cat'}
animals.add('fish')
animals.remove('cat')
animals.add('dog')  # does nothing and no return
print('dog' in animals)  # True
print('do  g' in animals)  # False
print(len(animals))  # 3

animals = {'cat', 'dog', 'fish'}
for idx, animal in enumerate(animals):
    print('#%d: %s' % (idx + 1, animal))

from math import sqrt
nums = {int(sqrt(x)) for x in range(30)}
print(nums)  # {0, 1, 2, 3, 4, 5}

d = {(x, x + 1): x for x in range(3)}
t = (0, 1)
print(d)  # {(0, 1): 0, (1, 2): 1, (2, 3): 2}
print(type(t))  # <class 'tuple'>
print(type(d[t]))  # <class 'int'>
print(d[t])  # 0
print(d[(1, 2)])  # 1

• 函数

def sign(x):
    if x > 0:
        return 'positive'
    elif x < 0:
        return 'negative'
    else:
        return 'zero'

for x in [-1, 0, 1]:
    print(sign(x))

negative
zero
positive

def hello(name, loud = False):
    if loud:
        print('HELLO,%s !' % name.upper())
    else:
        print('Hello,%s' % name)

hello('Bob')  # Hello,Bob
hello('Fred', loud=True)  # HELLO,FRED !

• 类

class Greeter(object):
    def __init__(self, name):
        self.name = name

    def greet(self, loud=False):
        if loud:
            print('HELLO, %s!' % self.name.upper())
        else:
            print('Hello,%s' % self.name)

g = Greeter('Fred')
g.greet()  # Hello,Fred
g.greet(loud=True)  # HELLO, FRED!