website/content/blog/pydecorators.md

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2020-03-30 22:20:38 +00:00
---
title: "Quick Python: Decorators"
date: 2020-03-30T18:07:14-04:00
draft: false
tags: ["python"]
---
[Geir Arne Hjelle](https://realpython.com/team/gahjelle/) at Real Python wrote a great post called [Primer on Python Decorators](https://realpython.com/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
```python
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...
```python
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.
```python
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}!"
```