Python 3 conditions, loops and assert, pass, Del

Conditions:

If condition:
Statement block
Elif
Statement block
Else
Statement block

Elif indicates else if

This is actually legal!!! 1 < x < 2!!!

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1. >>> If 1 < x < 2:
2. print (' True ')
5. True

and represents and

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1. >>> if x > 1 and x < 2:
2. print (' True ')
5. True

or represents or

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1. >>> x
2. 2
3. >>> if x = = 2 or x = = 3:
4. print (x)
7. 2

Returns a If B is true, otherwise returns C

A If B else c

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1. >>> ' True ' if 1 < x <2 else ' False '
2. ' True '

While loop

While condition:

Statement block

No brackets required!

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1. >>> x
2. 1.2
3. >>> while x < 2:
4. print (x)
5. x + = 0.2
8. 1.2
9. 1.4
10. 1.5999999999999999
11. 1.7999999999999998
12. 1.9999999999999998
13. >>>

Often used:

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1. While True:
2. ....
3. If ...:
4. Break
5. ....

For loop

For something in XXXX:

Statement block

That is, for each element in XXXX, execute some block of statements, XXXX can be a list, a dictionary, a tuple, an iterator, and so on.

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1. >>> for x in range (0,10):   
2.     print (x*x)   
3. &NBSP;&NBSP;
4.       
5. 0&NBSP;&NBSP;
6. 1&NBSP;&NBSP;
7. 4&NBSP;&NBSP;
8. 9&NBSP;&NBSP;
9. 16&NBSP;&NBSP;
10. 25&NBSP;&NBSP;
11. 36&NBSP;&NBSP;
12. 49&NBSP;&NBSP;
13. 64&NBSP;&NBSP;
14. 81&NBSP;&NBSP;

This is for: else ... Statement
Execute only without break, or, as long as you do not break, it will execute

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2. >>> for n in range (81,-1):
4. root = sqrt (n)
6. if root = = Int (root):
8. print (n)
10. Break
12. Else
14. print ("I didn ' t fint it")
20. I didn ' t fint it

But you should use the list derivation as much as possible, as it is more convenient and clear

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2. >>> [x*x for x in range (1,5)]
4. [1, 4, 9, + ]
6. >>> [x**2 for x in range (1,Ten) if x 2 = =0] /c5>
8. [4, +, + , + ]
10. >>> [(x, y) for x in range (1,3) for y in range (4,6)]
12. [(1, 4), (1, 5), (2, 4), (2, 5)]

Assert assert
The following statement is true, otherwise assertionerror appears

Used to check a condition, and if it is true, do nothing. If it is false, it throws a Asserterror and contains an error message.

For example:

py> x =23py> assertx > 0, "x is not zero or negative"py> assertx%2==0, "x is not an even number"Traceback (most recent call last):File"", line 1, inAssertionError: x isnotan even number

#常用在代码开头的注释 assert target  in (x, y, z) if target  = = x:      run_x_code() elif target  = = y:      run_y_code() else :      assert target  = = z      run_z_code()

Pass

Pass means there's nothing here, no action done.

If your program has unfinished functions and classes and so on, you can add some comments first, then the Code section just write a pass, so the program can run without error, and later you can continue to refine your program

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1. >>> class Nothing:
2. Pass
4. >>>

Del
Del deletes only references and names, does not delete values, that is, Python automatically manages memory and is responsible for memory recycling, which is one reason why Python is running less efficiently.

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2. >>> x = [1,2,3]
4. >>> y = x #x and y point to the same list
6. >>> del x
8. >>> x
10. Traceback (most recent):
12. File "<pyshell#41>", line 1, in <module>
14. X
16. Nameerror:name ' x ' is not defined
18. >>> y
20. [1, 2, 3]
    

Python 3 conditions, loops and assert, pass, Del