使用列表理解的Python方式

Question

I have this code that I know can be much more efficient....

return_list = []
for attribute in attributes:
    if attribute['internalname'] in attributenames:
        attr = Attribute();
        attr.name = attribute['internalname']
        attr.value = attribute['value']
        return_list.append(attr);

return return_list

I could have easily done list comprehension but since there is an object creation attr=Attributes() , I don't know how would I handle that?

Answer 1

Make Attribute() take the name and value as arguments:

return [Attribute(attr['internalname'], attr['value'])
        for attr in attributes if attr['internalname'] in attributenames]

You could make it accept keyword arguments instead if you need to support various attributes instead:

return [Attribute(name=attr['internalname'], value=attr['value'])
        for attr in attributes if attr['internalname'] in attributenames]

If neither is an option, make a function to create the Attribute object with a name and value attribute:

def create_attribute(name, value):
    attr = Attribute()
    attr.name = name
    attr.value = value
    return attr

return [create_attribute(attr['internalname'], attr['value'])
        for attr in attributes if attr['internalname'] in attributenames]

Answer 2

Is there an __init__ method for Attribute that takes named parameters? If so, you could do something like

return_list = [Attribute(name=attribute['internalname'], value=attribute['value']) for attribute in attributes if attribute['internalname'] in attributenames]

Answer 3

This is meant as a comment, posting as an answer because the comment system is not suited for posting code samples. There are two good answers using list comprehensions and I want to complement talking about generators and generator expressions.

While a list comprehension is shorter than your function, it may also be less readable. An alternative is using generators:

def make_attributes(attributes, attributenames):
    for attribute in attributes:
        if attribute['internalname'] in attributenames:
            attr = Attribute();
            attr.name = attribute['internalname']
            attr.value = attribute['value']
            yield attr

There is also generator expressions (use these in place of list comprehensions where you don't really need a real list, for example, when you are just iterating over the result):

# borrowing create_attribute from Martijn Pieters
return (
    create_attribute(attr['internalname'], attr['value'])
    for attr in attributes 
    if attr['internalname'] in attributenames
)