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content/blog/pydecorators.md

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+---
+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}!"
+```
+