Python，0a

 1 # Arithmetic expressions - numbers, operators, expressions
 2 
 3 print 3, -1, 3.14159, -2.8
 4 
 5 # numbers - two types, an integer or a decimal number
 6 # two corresponding data types int() and float()
 7 
 8 print type(3), type(3.14159)
 9 print type(3.0)
10 
11 
12 # we can convert between data types using int() and float()
13 # note that int() takes the "whole" part of a decimal number and doesn‘t round
14 # float() applied to integers is boring
15 
16 print int(3.14159), int(-2.8)
17 print float(3), float(-1)
18 
19 
20 # floating point number have around 15 decimal digits of accuracy
21 # pi is 3.1415926535897932384626433832795028841971...
22 # square root of two is 1.4142135623730950488016887242096980785696...
23 
24 # approximation of pi, Python displays 12 decimal digits
25 
26 print 3.1415926535897932384626433832795028841971
27 
28 # appoximation of square root of two, Python displays 12 decimal digits
29 
30 print 1.4142135623730950488016887242096980785696
31 
32 # arithmetic operators
33 # +  plus  addition
34 # -  minus  subtraction
35 # *  times  multiplication
36 # /  divided by  division
37 # **    power  exponentiation
38 
39 print 1 + 2, 3 - 4, 5 * 6, 2 ** 5
40 
41 # Division in Python 2
42 # If one operand is a decimal (float), the answer is decimal
43 
44 print 1.0 / 3, 5.0 / 2.0, -7 / 3.0
45 
46 # If both operands are ints, the answer is an int (rounded down)
47 
48 print 1 / 3, 5 / 2, -7 / 3
49 
50 
51 # expressions - number or a binary operator applied to two expressions
52 # minus is also a unary operator and can be applied to a single expression
53 
54 print 1 + 2 * 3, 4.0 - 5.0 / 6.0, 7 * 8 + 9 * 10
55 
56 # expressions are entered as sequence of numbers and operations
57 # how are the number and operators grouped to form expressions?
58 # operator precedence - "please excuse my dear aunt sallie" = (), **, *, /, +,-
59 
60 print 1 * 2 + 3 * 4
61 print 2 + 12
62 
63 
64 # always manually group using parentheses when in doubt
65 
66 
67 print 1 * (2 + 3) * 4
68 print 1 * 5 * 4