Introduction to context manager in Python 3

fengbingchun 2021-10-29 06:14:49
introduction context manager python

      In any programming language , The use of resources such as file operations or database connections is common . But the supply of these resources is limited . therefore , The main problem is to ensure that these resources are released after use . If you don't release them , It will lead to resource leakage , And may cause the system to slow down or crash . If the user has a mechanism to automatically set and disassemble resources , This will be very helpful . stay Python3 in , You can use the context manager (context manager) To achieve , This helps to properly handle resources . The context manager is an object .

      stay Python3 in , You can manage resources through the context manager . There are three ways to create a context manager : Class based context manager 、 Generator based context manager and function decorator based context manager .

      When creating a context manager using a class , Users need to ensure that this class has __enter__() and __exit__() Method .__enter__() Return the resources to be managed ; and __exit__() Don't return anything , Instead, perform cleanup operations .

      Context managers usually use with The statement calls , But they can also be used by calling their methods directly . The context manager defines the context in which with The runtime context to build at statement time , It handles incoming (entry into) And exit (exit from) The required runtime context to execute the code block .

      The most common way to perform file operations is to use with keyword .Python Of with Statement supports the concept of runtime context defined by the context manager . This is using a pair of methods (__enter__, __exit__) Realized , These methods allow user-defined classes to enter the runtime context before the statement body is executed and exit at the end of the statement .

      with grammar :


      with The execution of the statement is as follows

      (1). Evaluate the context expression to get the context manager ;

      (2). Of the context manager __enter__() Loaded for later use ;

      (3). Of the context manager __exit__() Loaded for later use ;

      (4). Of the context manager __enter__() Method is called ;

      (5). If TARGET Included in with In the sentence , Will __enter__() Assign to it the return value of .with The statement guarantees if __enter__() Method returns no error , that __exit__() Will always be called . therefore , If assigned to TARGET An error occurred during the list , It will be seen as SUITE What happened in .__enter__() Methods can either have a return value or no return value , If there is no return value, it will return... By default None.

      (6).SUITE Be performed ;

      (7). Of the context manager __exit__() Method is called . If the exception causes SUITE sign out , Then its type 、 Value and backtracking (traceback) Pass as a parameter to __exit__(); otherwise , Pass on 3 individual None.__exit__() Methods can also have return values , This return value should be a boolean type True or False, The default is None( namely False). If False, Exception will be thrown , Users need to handle exceptions . If True, The exception is ignored .

      If SUITE Exit due to exception , also __exit__() The value returned by the method is False, The exception is triggered . If the return value is True, The anomaly is suppressed , And continue with Statement after statement .

      If SUITE Exit for any reason other than exception , be __exit__() The return value of will be ignored , And continue at the normal position of the exit type taken .

      contextmanager.__enter__(): Enter the runtime context and return this object or another object related to the runtime context . The value returned by this method is bound to... Using this context manager with Of the statement as The target specified in clause ( If there is ). An example of a context manager that returns itself is a file object . File object from __enter__() Return to itself to allow open() Used as a with The context expression in the statement .

      contextmanager.__exit__(exc_type, exc_val, exc_tb): Exits the runtime context and returns a boolean flag , Indicates whether any exceptions that occur should be suppressed . If in execution with An exception occurred while the body of the statement , The parameter contains the exception type 、 Values and backtracking information (traceback information). otherwise , All three parameters are None. Returns... From this method True Will lead to with Statement suppresses the exception and continues execution immediately after with Statement after statement . otherwise , After this method is executed , The exception will continue to propagate . Exceptions that occur during the execution of this method replace with Any exception that occurs in the body of the statement . The passed in exception should not be explicitly re thrown , contrary , This method should return False To indicate that the method completed successfully and did not want to suppress the thrown exception . This allows context management code to easily detect __exit__() Whether the method actually failed .

      Typical uses of the context manager include saving and restoring various global states 、 Lock and unlock resources 、 Close open files, etc .

      When to use the context manager : When your code must open and close connections as part of its process ; When your code must use preconditions and postconditions (preconditions and post-conditions) when ; And when you try to manage limited resources on the system , Context manager is a good choice .

      The above content mainly refers to :



      Here is the test code :

import contextlib
var = 3
if var == 1:
# reference:
# Class based context manager
class FileManager():
def __init__(self, filename, mode):
self.filename = filename
self.mode = mode
self.file = None
def __enter__(self):
self.file = open(self.filename, self.mode)
return self.file
def __exit__(self, exc_type, exc_value, exc_traceback):
# loading a file
with FileManager('test.txt', 'w') as f:
print(f.closed) # True
elif var == 2:
# Generator based context manager : Instead of __enter__ and __exit__ Method , Generators and @contextlib.contextmanager The decorator will be yield Run everything before the statement , As if it were __enter__ Part of the method
# It's the same , The resulting value may be __enter__ Method will return the result . after , Will run with What's in the block , As a last step , Will run yield Code after statement , As if it were __exit__ The method is the same
def file_hanlder(file_name, file_mode):
file = open(file_name, file_mode)
print("open file")
yield file # yeild The previous code will be used in the context manager as __enter__ Method execution , And pass the results yield return , All in yield The following code will be used as __exit__ Method execution
print("close file")
with file_hanlder("test.txt", "w") as f:
print("write file")
print(f.closed) # True
elif var == 3:
# Context manager based on function decorator : One of the problems with the function decorator method is the inability to access __enter__ Return value of method
class file_handler_mixin(contextlib.ContextDecorator):
def __init__(self, file_name, file_mode):
self._file_name = file_name
self._file_mode = file_mode
self._file = None
def __enter__(self):
print(f"Enter Method: File Name {self._file_name}")
self._file = open(self._file_name, self._file_mode) # Cannot access file object from decorator function
return self._file
def __exit__(self,exc_type,exc_value,exc_traceback):
print(f"Exit Method: File Mode {self._file_mode}")
@file_handler_mixin("test.txt", "w")
def write_to_file():
print("Not access to the value returned by the __enter__ method")
print("test finish")



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