Recently, I have learned about the usage of with statement in python. It has some pretty interesting properties in the sense that it can be nicely used for contextual purposes. Acting very similarly as a @ decorator, the with is often used to execute codes before and after codes that it wrapped around. Consider this very basic example:
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class with_statement(object):
def __enter__(self):
print('Entering!')
def __exit__(self, exc_type, exc_val, exc_tb):
print('Exiting!')
with with_statement():
print('Inbetween string!')
So if you were to execute the codes above, here is the result you will see:
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Entering!
Inbetween string!
Exiting!
As you can see, when you wrapped the Inbetween string! print statement with the with with_statement():, it will execute the __enter__ codes first. Then your wrapped codes, then the __exit__ codes. So the basically idea is that it’ll execute the codes in those specific orders, which can be very powerful. To elaborate a bit more on this, here are some expanded idea on the previous codes that allows it to do a little bit more.
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class with_statement_two(object):
def __init__(self, dictKey):
self.dictKey = dictKey
def __enter__(self):
self.withDict = {}
for i in range(0, 5):
self.withDict[i] = 'I have %s cat(s)!' % i
print('Calculating how many cats I have...')
return self.withDict
def __exit__(self, exc_type, exc_val, exc_tb):
if self.dictKey in self.withDict:
print(self.withDict[self.dictKey])
with with_statement_two(2):
print('Inbetween string!')
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Calculating how many cats I have...
Inbetween string!
I have 2 cat(s)!
So in this scenario, we can pass a parameter into the with_statement_two class object. With that parameter, you can see that the with_statement_two class object is using that parameter to search a dictionary it created and output a dictionary value accordingly. So in this scenario, a dictionary is created in the __enter__ function, meaning, the moment the with statement is called. Then, using the parameter it recieved upon its initialization of __init__, it searches the dictionary key for a dictionary value with the parameter passed in. In this case, I have 2 cat(s) is the resulting output because an integer of 2 is the parameter being passed through.
Another common example of using the with statement is with the native python functionality of opening text documents. So instead of the typing:
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myFile = open('fileName.txt', 'r')
for line in myFile:
do something
You can re-write it to be like this, which is considered to be a little bit cleaner!
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with open('fileName.txt', 'r') as myFile:
for line in myFile:
do something