Accessing object from static initializer in Python

Question

This follows on from a previous query of mine Static initializer in Python I am trying to find an object from one of it's properties as I would with a static constructor in java:

public class TestBuilder {
private String uid;
private String name;
private double speed;
private static ArrayList<TestBuilder> types = new ArrayList<TestBuilder>();
public static final TestBuilder SLEEPY;
public static final TestBuilder SPEEDY;

static {
    SLEEPY = new TestBuilder("1", "slow test", 500.00);
    SPEEDY = new TestBuilder("2", "fast test", 2000.00);
}

private TestBuilder(String uid, String name, double speed){
    this.uid = uid;
    this.name = name;
    this.speed = speed;
    types.add(this);
}

public static TestBuilder getBuilderFromName(String s){
    for (int i = 0; i < types.size(); i++){
        if (((TestBuilder) types.get(i)).name.equals(s)){
            return (TestBuilder) types.get(i);
        }
    }
    return null;
}

In python I have tried:

class TestBuilder:
    uid = str()
    name = str()
    speed = float()
    types = []

    def __init__(self, uid, name, speed):
        self.uid = uid
        self.name = name
        self.speed = speed
        self.types.append(self)

    @classmethod
    def lookupType(cls, name):
        for item in cls.types:
            if item.name == name:
                return cls

TestBuilder.SLEEPY = TestBuilder("1","slow test", 500.0)
TestBuilder.SPEEDY = TestBuilder("2","fast test", 2000.0)

Then in a test module:

class Tester(unittest.TestCase):
    def test_builder(self):
        dummy = TestBuilder
        ref = dummy.SPEEDY
        objfound = dummy.lookupType("slow test")
        logger.debug("--test_builder() name: "+objfound.name)

While the correct object is found in the lookupType method the resultant classfound name property is empty when accessed in the test. How can I access the correct TestBuilder object given one of it's properties?

Martijn Pieters · Accepted Answer · 2014-12-14 03:01:47Z

3

You are returning the class, not the instance you found:

@classmethod
def lookupType(cls, name):
    for item in cls.types:
        if item.name == name:
            return cls
            #      ^^^

Return the item instead:

return item

Instead of using a list, register your classes using a dictionary so you can simply look them up by name:

class TestBuilder:
    types = {}

    def __init__(self, uid, name, speed):
        self.uid = uid
        self.name = name
        self.speed = speed
        self.types[name] = self

    @classmethod
    def lookupType(cls, name):
        return cls.types.get(name)

Note that your uid = str() and speed = float() lines simply create class attributes set to an empty string and 0.0 respectively. Those lines do not declare a type and Python won't stop your code from assigning different objects to those attributes at the instance level.

edited Dec 14, 2014 at 3:01

answered Dec 14, 2014 at 2:26

Martijn Pieters

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8 Comments

Don Smythe Over a year ago

Much appreciated. NB I changed the types declaration to types = {}

Martijn Pieters Over a year ago

@DonSmythe: oops, that's a typo on my part, corrected.

Yani Over a year ago

I would suggest using a variable name other than types, as it is the name of a module within the standard Python library.

Martijn Pieters Over a year ago

@Yani: that hardly matters here; it is a class attribute here, so it is namespaced.

Yani Over a year ago

@MartijnPieters - True, but it's good practice in my opinion.

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