Python small white (no programming foundation, no computer basis) development of the Road auxiliary knowledge 1 with...as

This syntax is used instead of the traditional try...finally syntax.

With EXPRESSION [as VARIABLE] With-block

The basic idea is that the object with which the value is evaluated must have a __enter__ () method, a __exit__ () method.

Immediately after the statement that follows with is evaluated, the __enter__ () method of the returned object is called, and the return value of the method is assigned to the variable following the AS. The __exit__ () method of the previous return object is called when all code blocks following the with are executed.

[Python]

1. File = open ("/tmp/foo.txt")
2. Try
3. data = File.read ()
4. Finally
5. File.close ()

Use with...as ... Replace the modified code with the following:

[Python]

1. With open ("/tmp/foo.txt") as File:
2. data = File.read ()

[Python]

2. #!/usr/bin/env python
4. # with_example01.py
10. Class Sample:
12. def __enter__ (self):
14. print ( "in __enter__ ()")
16. return "Foo"
20. def __exit__ (self, type, value, trace):
22. print ( "in __exit__ ()")
28. Def get_sample ():
30. return Sample ()
36. With Get_sample () as Sample:
38. Print ( "Sample:", sample)

Execution results are [Python]

1. In __enter__ ()
2. Sample:foo
3. In __exit__ ()

1. The __enter__ () method is executed

2. The value returned by the __enter__ () method-This example is "Foo", assigned to the variable ' sample '

3. Execute code block, print variable "sample" with the value "Foo"

4. The __exit__ () method is called with the real power that it can handle exceptions. You may have noticed that the __exit__ method of the sample class has three parameters-Val, type, and trace. These parameters are quite useful in exception handling.

Python small white (no programming foundation, no computer basis) development of the Road auxiliary knowledge 1 with...as