Python - Foundation

https://docs.python.org/3/library/index.html

-1. print format and f string, use print()

eg: 
    x='seven'
    print('x is {}'.format(x))
    print ('x is {} {}'.format (8,10))
    print ('x is {1:<09} {0:>09}'.format (8,10))
    a=8
    b=10
    x= f'seven {b} {a}'
    print (x)

-2. decimal

eg:
    from decimal import *
    a = Decimal('.10')
    b = Decimal('.30')
    x = a+a+a-b
    print (x) -->0.00
if no decimal, x is almost 0 but not 0 due to accuracy and precision

-3. / or //

eg:
    7/3 =2.3333333333333335
    7//3 =2
    7%3 =1

-4. type() and id(), isinstance()

eg:
    x=(1,2,3)
    print (type(x))

    print (id(x))

    print (isinstance(x, turple))

-5. if

eg:
    hungry=0
    x="feed" if hungry else "not feed"
    print (x)

-6. args

eg:
    def kittern(*args):
        if len(args):
            for s in args:
                print (x)
        else: print "no args"

    def main():
        x=(1,2,3)
        kittern (*x)

-7. keyword args

eg:
    def kittern(**kwargs):
        if len(kwargs):
            for k in kwargs:
                print (k, kwargs[k])
        else: print "no kwargs"

    def main():
        x=dict(a=1,b=2,c=3)
        kittern (**x)

-8. generators, yield ?

eg:
    range(10)

    yield 

-9. decorators

eg:
    def f1(f):
        def f2():
            print ("before")
            f()
            print ("after")
        return f2

    @f1
    def f3():
        print("this is f3")

    f3()

-10. list comprehension

eg:
    seq1=range(10)
    seq2=[x *2 for x in seq if x%3!=0]
    seq3=[(x,x**2) for x in seq]
    seq4=[ round(pi,i) for i in seq]
    seq5={x:x**2 for x in seq}
    seq5={x:x**2 for x in seq}

-11. init() and str()

-12. python的各种下划线 >>https://zhuanlan.zhihu.com/p/36173202 >>https://aji.tw/python%E4%BD%A0%E5%88%B0%E5%BA%95%E6%98%AF%E5%9C%A8__%E5%BA%95%E7%B7%9A__%E4%BB%80%E9%BA%BC%E5%95%A6/ >>https://www.cnblogs.com/coder2012/p/4423356.html

-13. Exceptions

eg:
import sys

    try:
        x=ini('foo')
    except ValueError as e:
        print('caught a value err')
        print(f'err:{e}')
    except ZeorDivisionError:
        print('caught a value err')
    except:
        print(f'unkonw error:' {sys.exc_info()})
    else:
        print('good job')

-14. raise errs

eg:
    if arg1<1:
        raise TypeError(f'excepted at least 1 argement')

-15. file I/O

eg:
    rb, wb
    rt, wt

-16. built-in function

eg:
    zip()
    enumerate()

-17. DB

eg:
    import sqlite3

    db=sqlite3.connect('db-api.db')
    cur=db.cursor()
    cur.execuret(""" Insert into... """
    db.commit()
    cur.execute("Select ...")
    db.close()

-18. global vs local

-19. del, 删除的是变量，而不是数据

if __name__=='__main__':
    a=1       # 对象 1 被 变量a引用，对象1的引用计数器为1
    b=a       # 对象1 被变量b引用，对象1的引用计数器加1
    c=a       #1对象1 被变量c引用，对象1的引用计数器加1
    del a     #删除变量a，解除a对1的引用
    del b     #删除变量b，解除b对1的引用
    print(c)  #最终变量c仍然引用1

-20. 循环 enumerate()

days=['Mon','Tue','Wed','Thu','Fri','Sat','Sun']
for i, d in enumerate(days):
    print(i,d)