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66 lines
1.7 KiB
Markdown
66 lines
1.7 KiB
Markdown
---
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title: "Quick Python: Decorators"
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date: 2020-03-30T18:07:14-04:00
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draft: false
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tags: ["python"]
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---
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[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.
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I find decorators useful for several reasons
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- Check a pre-existing condition (Is the user logged in?)
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- Perform post processing on function output (Convert to SI units)
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- Expose extra variables for use in the function
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Here is a template for how a decorator is written
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```python
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import functools
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def decorator(func):
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@functools.wraps(func)
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def wrapper(*args, **kwargs):
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# Do something before
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value = func(*args, **kwargs)
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# Do something after
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return value
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return wrapper
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```
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If your decorator takes arguments then there's another layer...
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```python
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def decorator_with_argument(argument):
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def decorator(func):
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@functools.wraps(func)
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def wrapper(*args, **kwargs):
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# Do something before
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value = func(*args, **kwargs)
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# Do something after
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return value
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return wrapper
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return decorator
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```
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## Example: Logging
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Let's write a decorator that logs the string returned by the function to a file.
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```python
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def filelog(filename):
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def decorator(func):
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@functools.wraps(func)
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def wrapper(*args, **kwargs):
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with open(filename, 'w') as f:
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f.write(func(*args, **kwargs))
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return wrapper
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return decorator
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@filelog('log.txt')
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def greet(name):
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return f"Hello {name}!"
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```
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