Using first-class objects and function names f formatter recursive iterator

1. The first-class objects and use the function name

Function name can be assigned as a value

def func():
    print(1)
print(func)
a = func
a()                   # <function func at 0x000001A24F8E9B70>     1

Function name can be used as an element stored in the container

def func():
    print(1)
lst = [func,func,func]
for i in lst:
    i()                 # 1 1 1

Function name can be used as an argument to another function

def func(f):
    f()
def foo():
    print(123)
func(foo)                         # 123

It can be used as the return value of the function name

def func():
    def foo():
        print(123)
    return foo
a = func()
a()                                # 123

2.f format

F formatted role

Filling string

s = f"你好{'常鑫'}"
s1 = F"{你好'常鑫'}"

Variable filling

s = "常鑫"
S1 = f"你还是挺好的{s}"
print(s1)

Filling formula

s1 = "{35+15}"
print(s1)

Filled with expressions

a = 10
b = 20
s1 = f"{a if a>b else b}"
print(s1)

Filled with braces

s1 = f"{{{{{{'常鑫'}}}}}}"
print(s1)            # {{{'常鑫'}}}

3. iterator

Iterators, a tool

Iterables:

list,dict,str,set,tuple -- 可迭代对象 使用灵活

Check whether the iteration

method one:

list.__iter__()

Second way:

View source

Three ways:

print(dir(list))

As long as official statements have __iter __ () method is iterable

Advantages and disadvantages

1. Advantages

1.1 flexible

1.2 Direct View value

2. shortcomings

2.1 consumes memory

Iterator

As long as official statements have __iter __ () method __next __ () method is iterator

句柄"f"是个迭代器
f = open("xxxx","w")
f.__iter__()
f.__next__()

lst = [1,2,3,4,6]
new_list = lst.__iter__()  #将可迭代对象转换成迭代器
new_list.__iter__()
new_list.__next__()

s = "123434"
new_s = s.__iter__()       #将可迭代对象转换成迭代器
print(new_s)
new_s.__iter__()
new_s.__next__()

new_s = s.__iter__()       #将可迭代对象转换成迭代器
print(new_s)
new_s.__iter__()
print(new_s.__next__())
print(new_s.__next__())
print(new_s.__next__())

s = "12343"               # 更改版for的本质
s = [1,2,3,4,5,7]
count = len(s)
new_s = s.__iter__()
while count:
    print(new_s.__next__())
    count -= 1

s = "12345"
new_s = s.__iter__()
while True:
    try:
        print(new_s.__next__())   # for真实本质
    except StopIteration:
        break
        
# except Exception:
#     print("我是万能的!")
#     break

4. recursive

definition

1. calls itself (continue to call themselves) - Dead recursion

2. there is a clear termination condition

Meet the above two is valid recursive

Delivery: execution conditions have been met until the end

Return: From the beginning of the end of back-off condition

Official statement: maximum level of 1000, the actual test 998/997

示例:      猜3
def age(n): # 1,2,3
    if n == 3:
        return "猜对了"
    else:
        return age(n+1)
print(age(1))

break down

def age2(n):
    if n == 3:
        return "猜对了"

def age1(n):
    if n == 3:
        return "猜对了"
    else:
        age2(n+1)

def age(n):
    if n == 3:
        return "猜对了"
    else:
        age1(n+1)
age(1)

to sum up

Iterables:
Advantages: flexible, you can directly see the value of
disadvantages: total memory, not the value of the iteration

Iterator:
Advantages: save memory, inert mechanism
Disadvantages: not flexible, the operation is more complicated, can not see the elements directly

Iterator characteristics:
1. disposable (no run out)
2 can not retrograde (not reverse)
3. an inert mechanism (save memory)

What is iterables:
has many proprietary methods, with __iter __ () method is a iterable

What is an iterator:
has __iter __ () and __next __ () method is iterator

When using iterators: when a container large amount of data when using an iterator