How to Use contextmanager Decorator To Create Custom with Statements in Python

The aim of this page📝 is to describe the creation of a context manager on the example of a function that goes to a specified folder ⟹ executes a random function ⟹ returns to where it started — without having to handle the journey there and back each time manually

Context manager is a built-in Python protocol implemented via the with statement

• In Python, we have the with statement that replaces the need to use pair statements ...such as open() and close() for file handling
• This is part of a protocol known as context manager

protocol is a set of operations that a type must support to implement

• For the behavior of context protocol, see ..With Statement < Compound statements < Python documentation ..PEP 343 — The “with” Statement | peps.python.org
• The essential behavior is that

context managers provide __enter__() and __exit__() methods that are invoked on entry to and exit from the body of the with statement.

Context Library built-in module contains a contextmanager decorator that can be used to implement a custom context manager

• context manager is also a function of context library module

@contextlib.contextmanager¶ This function is a decorator that can be used to define a factory function for with statement context managers, without needing to create a class or separate __enter__() and __exit__() methods.While many objects natively support use in with statements, sometimes a resource needs to be managed that isn’t a context manager in its own right, and doesn’t implement a close() method for use with contextlib.closing

• The structure of the custom context manager built with the @contextmanager decorator looks like this

• When in doubt about decorators see Explaining Function Decorators in Python

My implementation of the built-in context manager function

• In scripting/just CLI work (powershell/bash) it is typical that you need to go somewhere, do something, and return to the point of departure
• In PowerShell, this is done with the pair function push-location and pop-location
• This is the final implementation — ignore the first function that’s also a decorator I’m using to abstract away git for operational purposes …e.g. access provision to AWS subaccounts
• What matters here is the push_and_pop_path function and its structure which allows it to ...1.go to the repo, check if it's dirty, pull changes if needed, checkout the branch ...2.return ...3.run the function which in this case populates iam template for AWS access and copies the file to the repo with the checked-out branch ...4.to to the repo again, commit and push changes and ask if a PR should be opened

The whole story belongs to the wider epic of side-effects functions — functions, that come in pairs

• Uncle Bob has a nice rant about this

how good at we at managing pairs of functions like this? 
for example how good are we at managing malloc and free 
the answer to that is that we're terrible at managing that 
the obvious evidence of our terrible ability to manage pairs of functions is that 
in modern languages, we've invented a horrible hack to allow us to forget about managing pairs of functions 
that horrible hack is called garbage collection 

now I would not want to program without garbage collection   
because garbage collection makes it much easier to write safe code 
but you must admit that garbage collection is a crutch 
it is not reasoned 
you did not write the code in a reasoned way 
you  did not free everything you allocated 
instead what you did is say - the system will take care of it 
and ok fine we have written this horrible hack we've declared that we're  
going to depend upon it we've acknowledged that we need the crutch 

but allocation and freeing are not the only side effect function 
there are many other side effect functions  that you and I have to deal with 
like open and close 

does anybody seen a system crash because too many people forgot to close files 
yes and you leave a bunch of file descriptors open in the operating system 
and eventually, you run out of file descriptors that's called a file descriptor leak 
has anybody seen a system crash because all the graphics contexts got leaked 
or the semaphores didn't get closed 

anything that comes in a pair like that will suffer the same fate that memory used to suffer 
when we had memory leaks (and by the way you can still have memory leaks in Java)

— https://youtu.be/7EmboKQH8lM?t=5252

LINKS

• https://stackoverflow.com/questions/299446/how-do-i-change-directory-back-to-my-original-working-directory-with-python
• https://youtu.be/7EmboKQH8lM?t=5252
• Protocols in Python | by Pavol Kutaj | Medium
• With Statement < Compound statements < Python documentation
• Explaining Function Decorators in Python