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)
- 十六进制:
- 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
- 次方:
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
- 使用
-
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)
- 首字母大写:
-
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
- 使用
- 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
: readw
: writer+
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 errorexcept
:block of code will be executed in case there is an error intry
blockfinally
: 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()