Quick Python: Decorators
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Geir Arne Hjelle at Real Python wrote a great post called Primer on Python Decorators. I recommend reading that as this post serves mostly as a reminder to myself on how to write a decorator.
I find decorators useful for several reasons
- Check a pre-existing condition (Is the user logged in?)
- Perform post processing on function output (Convert to SI units)
- Expose extra variables for use in the function
Here is a template for how a decorator is written
import functools
def decorator(func):
@functools.wraps(func)
def wrapper(*args, **kwargs):
# Do something before
value = func(*args, **kwargs)
# Do something after
return value
return wrapper
If your decorator takes arguments then there’s another layer…
def decorator_with_argument(argument):
def decorator(func):
@functools.wraps(func)
def wrapper(*args, **kwargs):
# Do something before
value = func(*args, **kwargs)
# Do something after
return value
return wrapper
return decorator
Example: Logging
Let’s write a decorator that logs the string returned by the function to a file.
def filelog(filename):
def decorator(func):
@functools.wraps(func)
def wrapper(*args, **kwargs):
with open(filename, 'w') as f:
f.write(func(*args, **kwargs))
return wrapper
return decorator
@filelog('log.txt')
def greet(name):
return f"Hello {name}!"