Python 语法速查(一)

Printing

• 链接字符串

a="abc"
b="123"
print(a+b)

• 使用,输出空格

print("Hens", 25+30/6); #Hens 30
print("Hens", "Kay", "Thomas"); #Hens Kay Thomas
print("Is it greater or equal?", 5 >= -2) #Is it greater or equal? True

• 格式化字符串
  • 使用format

    age_eval = "my age is {}"
    print(age_eval.format(age)) #my age is 33
    print("This is a string {}".format('INSERTED'))
    print("The {} {} {} ".format(1,2,3))
    print("The {0} {0} {0}".format(20.3,100)) #The 20.3 20.3 20.3
    print("The {q} {b} {f}".format(f='fox', b='brown', q='quick')) #The quick brown fox
    #格式化浮点数:{value:width.precision f}
    result = 100/777 #0.1287001287001287
    print("result is {r:1.3f}".format(r=result)) #result is 0.129
  
  

  • 使用f-string

    name="tao"
    age = 33;
    #使用fstring
    print(f"{name} is {age} years old ") #my name is tao
  

Primary Types

• locals()/globals()

#查看全局变量
print(globals())
#{'__name__': '__main__', '__doc__': None, '__package__': None, '__loader__': <class '_frozen_importlib.BuiltinImporter'>, '__spec__': None, '__annotations__': {}, '__builtins__': <module 'builtins' (built-in)>}

def func:
    a = 'some text'
    b = 100
    #查看函数内的局部变量
    print(locals()) #{'b': 1, 'a': 's'}

Numbers

• 进制转换
  • 十六进制：hex(12)
  • 二进制：bin(1234)
• 内置数值运算
  • 次方:pow(2,4)等价2**4
  • 绝对值: abs(-2)
  • 四舍五入：round(3.9) #4.0

字符串

• 表示方式
  • s1 = 'string'
  • s2 = "string"
  • s3 = """string"""
  • 三者等价 s1 == s2 == s3 #True

• 支持[]索引

    a = "hello";
    a[0] #h
    a[-1] #o
  

• 字符串常量是Immutable,不能用[]的方式改变字符串内容

    s = 'hello'
    s[0] = 'H' #TypeError: 'str' object does not support item assignment
  
    s = 'H' + s[1:]
    s = s.replace('h', 'H')
  

• 格式化字符串

  • 使用fstring

      binary = "binary"
      do_not = "don't"
      y = f"Those who know {binary} and those who {do_not}."
    

  • 使用format

      id = 100
      name = 'kate'
      ss = 'no data available for person with id: {}, name: {}'.format(id, name)
    

• 获取字串
  • 使用[]索引,格式为[起始index:结束index:步长]
  • 左开右闭区间，类似C++中的迭代器

    a="hello"
    a[1:] #ello, 包括第一个字符
    a[:3] #hel, 不包括第三个字符
    a[1:3] #el
    a[1:-1] #ell，负数表示从后向前，-1表示倒数第1个字符l，因此区间为[1:4)
    a[:] #hello
    a[::] #hello
    a[::2] #hlo 步长是2，抽取字串
    a[1:-1:2] #el,起始1，终点-1，步长2
    a[::-1]#olleh, 反转字符串
  

• 分割字符串

    s = 'hello'
    s.split('e') #['h','llo']
    s.partition('l') #('he', 'l', 'lo')
  

• 其它API
  • 首字母大写: s.capitalize
  • 大小写转换: s.lower(),s.upper()
  • 字符出现次数:s.count('o')
  • 字符出现位置: s.find('o')
  • 检查字符是否是数字或字母: s.isalum()
  • 检查字符是否是字母:s.isalpha()
  • 开头结尾：s.startswith(str)/s.endswith(str)

• 转义字符

  Escape	What it does.
  \\	Backslash (\)
  \'	Single-quote (’)
  \"	Double-quote (”)
  \a	ASCII bell (BEL)
  \b	ASCII backspace (BS)
  \f	ASCII formfeed (FF)
  \n	ASCII linefeed (LF)
  \N	{name} Character named name in the Unicode database (Unicode only)
  \r	Carriage Return (CR)
  \t	Horizontal Tab (TAB)
  \uxxxx	Character with 16-bit hex value xxxx
  \Uxxxxxxxx	Character with 32-bit hex value xxxxxxxx
  \v	ASCII vertical tab (VT)
  \ooo	Character with octal value ooo
  \xhh	Character with hex value hh

Basic Statements

• And,Or,Not

1<2 and 2<3
2<3>10 #false, 等价于2<3 and 3>10
100==1 or 2==2
not 1==1

• if-elif-else

if some_condition: #注意冒号
    #execute some code 
elif some_other_condition:
    #do something different
else:
    # do something else

#三元运算
condition_is_true if condition else condition_is_false

• for

    list = [1,2,3]
    for item in list: #注意冒号
        print(item)
  
    #只关注循环次数
    for _ in list:
        print('cool')
  
    #遍历tuple list
    mylist = [(1,2),(3,4)]
    for t in mylist:
        print(t) #(1,2) (3,4)
    for (a,b) in mylist: #使用pattern matching
        print(a) #1 3
        print(b) #2 4
  
    #遍历map list
    d = {'k1':1, 'k2':2, 'k3':3}
    for item in d:
        print(item) #k1,k2,k3  #只返回key
  
    for item in d.items(): #返回key-value
        print(item) #('k1',1),('k2',2),('k3',3)
  
    for key,value in d.items(): #使用pattern matching
        print(value)
  

• while

    x = 0 
    while x<5:
        print(f'value of x is {x}')
        x += 1
    else:
        print('loop end')
  

• break/continue/pass
  • break: Breaks out of the current closest enclosing loop.
  • continue: Goes to the top of the closest loop.
  • pass: Does nothing at all.

• range

    for num in range(3,10,2):
        print(num) #打印3到9（不包括10），步长为2的数
    list(range(0,11,2)) #产生0-10的偶数
  

• enumerate

    word = 'adc'
    for item in enumerate(word): #返回一组tuple
        print(item) #(0,'a')(1,'b')(2,'c')
  
    for index,letter in enumerate(word):
        print(index)
        print(letter)
  

• zip

    list1 = [1,2,3]
    list2 = ['a','b','c']
  
    for item in zip(list1, list2):
        print(item) #(1,'a'),(2,'b'),(3,'c')
  
    list3 = list(zip(list1,list2))
  

• in

    2 in [1,2,3] #True
    'a' in 'world' #True
    'mykey' in {'mykey':345} #True
    d = {'mykey':345}
    345 in d.values() #True
  

• import

    from random import shuffle #从random库中引用shuffle函数
    list1 = [1,2,3]
    shuffle(list1)
  

• input

    result input('what is your name?') #从键盘接受输入到result，类型是string
    type(result) #str
    int(result)
    float(result)
  

数组

• 创建list

mylist=[1,2,'three']
mylist[2] #three
len(mylist) #长度
mylist[1] = 10
mylist.index(2) #1

• 追加元素

list1=['one','two']
list1.append('three') #['one', 'two', 'three']
list1.append([1,2]) #['one', 'two', 'three', [1, 2]]
list1.insert(2,'str') #['one', 'two', 'str', 'three',[1, 2]]

• 删除元素

val = list.pop() #默认删除尾部
val = list.pop(-1) #删除尾部
val = list.pop(2) # 删除index=2的元素
list1 = [1,2,2,3,4]
list1.remove(2) #删除数组中第一个2

• 拼接list

list1=[1,2,3]
list2=[4,5]
list3 = list1 + list2 #[1, 2, 3, 4, 5]
list1.extend(list2) #等价于list1 = list1+list2

• 其它API

list1=[1,2,3]
max_num = max(list)
min_num = min(list)
list1.revers()
list1.sort()

• functional API

mylist1 = [x for x in 'abc'] # ['a',b,'c']
mylist2 = [num**2 for num in range(0,11)] #[0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
mylist3 = [x for x in range(0,11) if x%2==1]#[1, 3, 5, 7, 9]
mylist4 = [x if x%2 == 0 else 'ODD' for x in range(0,11)] #[0, 'ODD', 2, 'ODD', 4, 'ODD', 6, 'ODD', 8, 'ODD', 10]
mylist5 = [x*y for x in [2,4,6] for y in [1,10,100]] #[2, 20, 200, 4, 40, 400, 6, 60, 600]

##返回一个参数为偶数的数组
def myfunc(*args):
    return [x for x in args if x%2 == 0]

Tuples

• Immutable，不能修改tuple中的元素
• Tuple uses parenthesis: (1,2,3)
• Only two methods
  • index
  • count

t=(1,2,3)
type(t) #tuple
t=('one',2)
t[0] #one
t[1] 2 #2
t=('a','a',2)
t.count('a') #2
t.index('a') #0
t[0]='NEW' #TypeError

字典(dict)

• 创建方式

d1 = {'name': 'jason', 'age': 20, 'gender': 'male'}
d2 = dict({'name': 'jason', 'age': 20, 'gender': 'male'})
d3 = dict([('name', 'jason'), ('age', 20), ('gender', 'male')])
d4 = dict(name='jason', age=20, gender='male')

• 无序字典

d={'k1':123, 'k2':[0,1,2], 'k3':{'insidekey':100}}
d['k2'] #[0,1,2]
d['k4']="abc"
keys = d.keys() 

Python 3.7后字典变成有序字典

• 访问
  • 使用[]访问，如果key不存在则抛异常
  • 使用get('key')访问，如果key不存在则返回默认值
• 删除
  • d.pop('key')
• 函数式API

#{key:value | 规则}
d = {x:x**2 for x in range(10)}#{0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81}
#{key:value | zip(k,v)}
d = {k:v**2 for k,v in zip(['a','b'],range(2))} #{'a': 0, 'b': 1}

• 按key/vlaue排序

d = {'b': 1, 'a': 2, 'c': 10}
d_sorted_by_key = sorted(d.items(), key=lambda x: x[0]) # 根据字典键的升序排序
d_sorted_by_value = sorted(d.items(), key=lambda x: x[1]) # 根据字典值的升序排序
d_sorted_by_key #[('a', 2), ('b', 1), ('c', 10)]
d_sorted_by_value #[('b', 1), ('a', 2), ('c', 10)]

集合(set)

• 无序哈希表，无法索引
• 创建

s1 = {1, 2, 3}
s2 = set([1, 2, 3])

• 添加元素

s1.add(1)
s1.add(2)

• 删除元素

s2.discard(2)
s2.remove(1)
s2.clear() 

• 检查元素存在

b1 = 1 in s1 #true
b2 = 10 in s2 #false

• 逻辑操作

#求差集
s1 = {1,2,3}
s2 = {2,3,4,5}
s2.difference(s1) #{2, 3, 4, 5}
s2.difference_update(s1) #s2更新为二者差集{2, 3, 4, 5}

#求交集
s1 = {1,2,3}
s2 = {1,2,4}
s1.intersection(s2) #{1,2}
s3 = {5,6}
s1.disjoint(s3) #无交集返回True，有交集返回False

#父集子集
s1.issubset(s2)
s1.issupperset(s1)

#求并集
s1.union(s2)
s1.update(s2) #将s1更新为s1,s2的并集

• 排序

s = {3, 4, 2, 1}
sorted(s) # 对集合的元素进行升序排序，返回一个array
#[1, 2, 3, 4]

文件操作

• mode
  • r: read
  • w: write
  • r+
  • w+
  • a : append

    myfile=open("./test.txt",mode='r') #_io.TextIOWrapper
    content = myfile.read() #string
    myfile.seek(0) #move file cursor to front
    myfile.readlines() #list
    myfile.close()
    file = open('TEXT.txt',mode='w')
    file.write('THis is a test file!')
    file.close()
  

• 使用with操作

    with open("welcome.txt") as file: # Use file to refer to the file object
        data = file.read()
        #do something with data
  

Lambda Expressions

• map/filter: 第一个参数是函数对象，第二个参数是数组

def square(num):
    return num*num
def check_even(num):
    return num%2 == 0

list1 = [12,3,3]
list2 = map(square,list1)
list3 = filter(check_even,list1)

• lamda expression

square = lambda num: num**2;
square(3)

mynums=[1,2,3,4]
list(map(lambda num:num**2,mynums))

Decorators

• 在不修改原函数的前提下，对已有函数进行扩展后，返回一个新的函数给原函数
• 在需要扩展的函数上面，使用@符号标记

def some_decorator(some_func)    
    def wrap_func()
        #some code
        some_func()
        #some code 
    return wrap_func            
    
@some_decorator 
def simple_func():
    #DO something
    return something

通过@修饰，将simple_func传递给some_decorator函数，some_decorator将wrap_func返回给simple_func，这样在后面调用simple_func时就相当于调用了wrap_func()

• Decorator的实现

def new_decorator(orig_func):
    def wrap_func():
        #some code before execute  orig_func
        print('before orig_func')
        orig_func()
        #some code after execute orig_func
        print('after orig_func')
    return wrap_func

def fun_needs_decorator():
    print("fun needs decorator")

fun_needs_decorator = new_decorator(fun_needs_decorator)
fun_needs_decorator()

#使用 @符号
@new_decorator
def fun_needs_decorator():
    print("fun needs decorator")

fun_needs_decorator()#得到相同结果

• Decorator可以嵌套使用

@my_decorator1
@my_decorator2
def func2(arg1, arg2):
    print(arg1, arg2)

OOP

• class，成员变量，成员函数

class NameOfClass(): #注意括号和冒号
    instance_variable = some_value #共有成员
    def __init__(self,param1, param2):
        self.param1 = param1 #定义私有成员
        self.param2 = param2
    def some_method(self): #成员函数
        print(self.param1)

class Dog():
    species="mammal" #定义共有成员变量
    def __init__(self,breed):
        self.name = breed #定义私有成员变量
    def bark(self): #定义成员函数
        print("WOOF") 

my_dog = Dog(breed='Lab')
type(my_dog)  #<class '__main__.Dog'>
print(my_dog.name)
print(my_dog.species)

• 继承

class Animal():
    def __init__(self):
        print("Animal created")
    def who_am_i(self):
        print("I am an animal")
    def eat(self):
        print("I am eating")

class Dog(Animal): #继承
    def __init__(self):
        Animal.__init__(self) #调用父类构造
        print("Dog Created")
    def who_am_i(self): #override
        print("I am a dog")

• 抽象类

class Animal():
    def __init__(self,name):
        self.name = name
    def speak(self):
        raise NotImplementedError("Subclass must implement this abstract method")

class Dog(Animal):
    def speak(self):
        return self.name + "say WOOF!"

• 特殊API

class Book():
    def __init__(self,title,author):
        self.title = title
        self.author = author
        self.pages = 100
    def __str__(self): #自定义类描述
        return f"{self.title} by {self.author}"
    def __len__(self): #len()使用
        return self.pages
    def __del__(self):
        print("A book object has been deleted")

b = Book('Python3','Jose')
print(b) #Python3 by Jose
len(b) #100
del b #A book object has been deleted

Modules and Packages

• 文件引用

### mymodule.py
def my_func():
    print("from my_module")

### other files
from mymodule import my_func
my_func()

• 文件夹(package)引用

└── package
├── __init__.py
├── main_script.py
└── subpackage
    ├── __init__.py
    └── sub_script.py

假设包结构如上，Python3中不再需要__init__.py

from package import main_script #引用包内文件
from package.subpackage import sub_script #引用包内文件

main_script.func_from_mainsript() #调用main_script的方法
sub_script.func_from_subsript() #调用sub_script的方法

• __main__

python没有main函数，当执行python xx.py时，在xx.py内有一个全局变量__name__被赋值为"__main__"表示这个文件是被直接运行的文件，也就是相当于main函数所在的文件。在程序里可以做如下判断:

if __name == '__main__':
    #当被直接运行时，需要执行的代码
    some_func()

Errors and Exception

• Three keywords
  • try: block of code might lead to an error
  • except:block of code will be executed in case there is an error in try block
  • finally: A final block of code to be executed, regardless of an error

def ask_for_int():
    while true:
        try:
            result = int(input("Please provide number: "))
        except:
            print("Whoops! this is not a number!)
            continue
        else:
            print(result)
            break
        finally:
            print("END")

• except可以捕获具体的错误类型

try:
    f = open('testfile','w')
    f.write("Write a test line")
except TypeError: #捕获具体错误类型
    print("There was a type error!")
except OSError: #捕获具体错误类型
    print("You have an OS Error")
finally:
    print("End)

Unit Test

• pylint: 静态语法检查
  • pip install pylint
  • > pylint xx.py
• unittest: built-in library，自带的单元测试库

#cap.py - file to be tested
def cap_text(str):
    return str.capitalize()
def title_text(str):
    return str.title()

#test.py - Unit test file
import unitest
import cap #file name

class TestCap(unittest.TestCase):
    def test_one_word(self):
            text = 'python'
            result = cap.cap_text(text)
            self.assertEqual(result,'Python')
    def test_multiple_words(self):
            text = 'monty python'
            result = title_text(text)
            self.assertEqual(result,'Monty Python')
    
if __name == '__main__':
        unittest.main()

Resources for basic Practice

• Intermediate Python
• Basic Practice
• More Mathematical (and Harder) Practice
• List of Practice Problems
• Reddit
• PythonChallenge