Python Learning notes-basic syntax

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1. Constants and variables

constant : Once assigned, it can no longer be changed, in other words, it cannot be re-assigned to a value. python does not exist constant

literal constants : A single occurrence of a quantity that is not assigned to any variable or constant

variable : is a name, on the left side of the assignment symbol, that name can refer to the right side of the assignment symbol

In [PNS]: i = 3In [+]: print (i) 3

2. System type

• Python is a strongly typed language--

• Python is a dynamic type language-- variables can be re-assigned to other types

  
  

n [MAX]: 3 + 3out[39]: 6In [+]: 3 + ' 3 '--------------------------------------------------------------------------- TypeError Traceback (most recent) <ipython-input-40-1142db5d42c6> in <mo Dule> ()----> 1 3 + ' 3 ' typeerror:unsupported operand type (s) for +: ' int ' and ' str ' in [all]: in [+]: i = 3In []: t Ype (i) out[42]: Intin [+]: i = ' 3 ' in [St]: Type (i) out[44]: str

3. Operators

3, 1 arithmetic operators-- arithmetic operators can only operate on int and float

In [the]: 1 + 1out[45]: 2In [1]: + 1.2out[46]: 2.2In []: 3/2out[47]: 1.5In []: 3//2out[48]: 1

3, 2 comparison operator-- The return value of the comparison operation is of type bool

In [$]: 3 > 5out[49]: Falsein [3]: 3 < 5OUT[50]: Truein [Wuyi]:! = 5out[51]: True

3/3 logical operators-- The operands of the logical operators are either bool types or types that can be implicitly converted to type bool, and the return value is of type bool

In [all]: True and falseout[52]: Falsein [+]: TRUE or falseout[53]: Truein [si]: not falseout[54]: True

4. Program Structure

4, 1 sequential structure

4/2 Branch Structure

Single Branch

In [all]: If a > 3: ...: Print (' a > 3 ') ...: print (a) ...: a > 35

Dual-branch structure

In [page]: a = 5In [+]: If a <: ...: print (' A < ') ...: else: ...: print (' a > Ten ') ...: P Rint (a) ...: a < 105

Multi-branch structure

in [+]: a = 30In []: If a <: ...: print (' A < ') ...: Elif a <: ...: print (' <= a < ') ...: else: ...: print (' a >= ') ...: print (a) ...: a >= 2030

4/3 Cycle Structure

While loop

In [a]: i = 0In [+]: While I < 5: ...: print (i) ...: i + = 1 ...: 01234

There must be some mechanism to modify the modulation so that it exits the loop, usually in the loop body to modify the condition

For loop

in [+]: For I in Range (0,5): ...: print (i) ...: 01234

Never modify an iterative object in a for in loop

5.Break and Continue

Break: Early termination

in [+]: For I in Range (0,10): ...: print (i) ...: If I% 2! = 0: ...: print (' OK ') ...: Break ...: 01ok

Continue: used to skip the remainder of the loop body

In [total]: For I in Range (0,10): ...: = if I% 2! = 0: ...: print (i) ...: Continue ...: 13 579

Else-- The ELSE clause is executed when the loop does not exit prematurely

In [66]: for i in range (0,5):    ...:      pass    ...: else:    ...:      Print (' OK ')     ...:     okin [67]: for i in  range (0,5):     ...:     break    ...:  else:    ...:     print (' OK ')     ..:          ...:     in [68]: for  i in range (0,5):    ...:     continue     ...: else:    ...:     print (' OK ')      ...:         ...:          ...:&nbSp;    ok 

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Python Learning notes-basic syntax