If you know what context managers are then you need nothing more to understand __enter__ and __exit__ magic methods. Lets see a very simple example.
In this example I am opening myfile.txt with help of open function. The try/finally block ensures that even if an unexpected exception occurs myfile.txt will be closed.
fp=open(r"C:\Users\SharpEl\Desktop\myfile.txt")
try:
for line in fp:
print(line)
finally:
fp.close()
Now I am opening same file with with statement:
with open(r"C:\Users\SharpEl\Desktop\myfile.txt") as fp:
for line in fp:
print(line)
If you look at the code, I didn't close the file & there is no try/finally block. Because with statement automatically closes myfile.txt . You can even check it by calling print(fp.closed) attribute -- which returns True.
This is because the file objects (fp in my example) returned by open function has two built-in methods __enter__ and __exit__. It is also known as context manager. __enter__ method is called at the start of with block and __exit__ method is called at the end. Note: with statement only works with objects that support the context mamangement protocol i.e. they have __enter__ and __exit__ methods. A class which implement both methods is known as context manager class.
Now lets define our own context manager class.
class Log:
def __init__(self,filename):
self.filename=filename
self.fp=None
def logging(self,text):
self.fp.write(text+'\n')
def __enter__(self):
print("__enter__")
self.fp=open(self.filename,"a+")
return self
def __exit__(self, exc_type, exc_val, exc_tb):
print("__exit__")
self.fp.close()
with Log(r"C:\Users\SharpEl\Desktop\myfile.txt") as logfile:
print("Main")
logfile.logging("Test1")
logfile.logging("Test2")
I hope now you have basic understanding of both __enter__ and __exit__ magic methods.