How python uses ContextLib
Editor to share with you how to use python ContextLib, I believe that most people do not know much about it, so share this article for your reference, I hope you can learn a lot after reading this article, let's go to know it!
Context Management Library (ContextLib)
The contextlib module contains tools related to the context manager and with declarations. Usually if you want to write a context manager, you need to define a class that contains enter methods as well as exit methods, such as:
Import timeclass demo: def _ init__ (self, label): self.label = label def _ enter__ (self): self.start = time.time () def _ exit__ (self, exc_ty, exc_val, exc_tb): end = time.time () print ('{}: {} '.format (self.label, end-self.start))
A complete example is here:
Import timeclass demo: def _ init__ (self, label): self.label = label def _ enter__ (self): self.start = time.time () def _ exit__ (self, exc_ty, exc_val, exc_tb): end = time.time () print ('{}: {} '.format (self.label, end-self.start) with demo (' counting'): n = 10000000 while n > 0: n-= counting: 1.36000013351
The context manager is activated by the with declaration, and this API involves two methods. The enter method, which executes when the execution flow enters the with code block. And it returns an object that can be used by the context.
When the execution flow leaves the with code block, the exit method is called, which cleans up the resources being used.
Rewrite the above example with the @ contextmanager decorator:
From contextlib import contextmanagerimport time@contextmanagerdef demo (label): start = time.time () try: yield finally: end = time.time () print ('{}: {} '.format (label, end-start)) with demo (' counting'): n = 10000000 while n > 0: n-= blank counting: 1.32399988174
Looking at the example above, all the code before yield in the function is similar to the content of the enter method in the context manager. All the code after yield is like the content of the exit method. If an exception occurs during execution, it is triggered in the yield statement.
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