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Patterns for Cleaner Python


  • asserts accept a second expression that can be used to print out useful info, eg. x=5; assert x > 10, f'{x} is not greater than 10' will raise AssertionError: 5 is not less than 10
  • Don't use asserts for data validation or catching run-time errors (since asserts can be globally disabled).
  • asserting a tuple will never result in an assertion error (tuples are truthy)
  • Use asserts as a debugging aid.

Context Managers

with open('hello.txt', 'w') as f:
    f.write('hello, world!')

# is almost equivalent to:
f = open('hello.txt', 'w')
    f.write('hello, world')

A context manager is a simple “protocol” (or interface) that your object needs to follow in order to support the with statement. Basically, all you need to do is add __enter__ and __exit__ methods to an object if you want it to function as a context manager. Python will call these two methods at the appropriate times in the resource management cycle.

There are two ways to support the with statement:

Class Based Context Managers

class ManagedFile:
    def __init__(self, name): = name
    def __enter__(self):
        self.file = open(, 'w')
        return self.file
    def __exit__(self, exc_type, exc_val, exc_tb):
        if self.file:


You can use the contextlib.contextmanager decorator to define a generator-based factory function for a resource that will then automatically support the with statement.