在用户定义的 class 中的任意属性上调用方法

Question

Ok, this is probably really simple but I'm struggling. So, let's say I have a simple class "Simple" that has some attributes a and b. And I have a method, "trim" that can be used on one attribute at a time:

class Simple():

    def __init__(self, a=None, b=None):

        self.a = a
        self.b = b

    def trim(self.[???]):
        ...do some stuff...
        return self

Now, I want the user to be able to apply the method to any of the attributes (ie, a and b) with something like:

simple_with_a_trimmed = simple.trim('a')

or

simple_with_b_trimmed = simple.trim('b')

How can I set up the method so that I can pass it the appropriate attribute? Thanks in advance for any thoughts!

Answer 1

You can use the get getattr and setattr methods. These allow you to refer to a class attribute by a string.

You can pass the desired attribute to the trim method. You can then retrieve the value of the specified attribute using getattr . Then, you can either transform it, and return it, or perhaps modify it using setattr -- to update the object.

Below is a simple example of this in action. The trim1 method merely returns the value of the (transformed) attribute but does not update the object. The trim2 method changes the object itself -- it updates the value of the specified attribute.

I've also added a __str__ method to show how the object changes after each call.

class Simple:

    def __init__(self, a=None, b=None):
        self.a = a
        self.b = b

    def trim1(self, attr):
        # Get value of the attribute
        current_value = getattr(self, attr)

        # Transform and return the value of the attribute
        # Removing whitespace as an example
        return current_value.strip()

    def trim2(self, attr):
        # Get value of the attribute
        current_value = getattr(self, attr)

        # Transform the value of the attribute
        # Removing whitespace as an example
        current_value = current_value.strip()

        # Update the value of the attribute
        setattr(self, attr, current_value)

    def __str__(self):
        return f'a: "{self.a}", b: "{self.b}"'

# Create an object of type Simple
simple = Simple('  data1', 'data2   ')

# These methods do not change the object, but return the transformed value
a = simple.trim1('a')
b = simple.trim1('b')
print(f'Returned value of a: "{a}"')
print(f'Returned value of b: "{b}"')

print(simple)  # Prints `a: "  data1", b: "data2   "`

# Apply the transformation to attribute `a`
simple.trim2('a')

print(simple)  # Prints `a: "data1", b: "data2   "`

# Apply the transformation to attribute `b`
simple.trim2('b')

print(simple)  # Prints `a: "data1", b: "data2"`