Python Morsels: Python's setattr function

Table of contents

1. Need to dynamically set an attribute?
2. Python's built-in setattr function to the rescue
3. Why not update __dict__ directly?
4. Why not call __setattr__?
5. Don't use setattr if you can avoid it
6. Use Python's setattr function for dynamic attribute assignment

Need to dynamically set an attribute?

We'd like to make a class that works like this:

>>> row=Row(id=4,name="duck",action="quack",color="purple")>>> row.id4>>> row.name'duck'

>>> row=Row(id=4,name="duck",action="quack",color="purple")>>> row.id4>>> row.name'duck'

Our Row class is supposed to accept any number of keyword arguments and assign each of them to an attribute on a new Row object.

We can use Python's ** operator to capture arbitrary keyword arguments into a dictionary:

classRow:def__init__(self,**attributes):forattribute,valueinattributes.items():...# What should we do now?

classRow:def__init__(self,**attributes):forattribute,valueinattributes.items():...# What should we do now?

But we need some way to store each item on our Row object as a new attribute.

Normally attribute assignments use an = sign with the . notation:

>>> row.color="purple"

>>> row.color="purple"

But we can't use the . notation because our attribute names are stored in strings. For example the variable attribute might contain the string "color" (representing the color attribute we're meant to assign to):

>>> attribute="color">>> value="purple"

>>> attribute="color">>> value="purple"

And if we try using Python's usual attribute assignment notation:

>>> row.attribute=value

>>> row.attribute=value

We'll end up with an attribute called attribute instead of an attribute called color:

>>> row.attribute'purple'

>>> row.attribute'purple'

We need some way to dynamically assign an attribute!

Python's built-in setattr function to the rescue

Python's setattr function accepts an …