CODEWITHSUNDEEP
pythondictionaryboolean
aoiee
aoiee

Reputation: 355

Python class instance evaluates to false?

In the following example:

class Example(dict):
  def __init__(self, name):
    self.name = name

  def get_name(self):
    return self.name

ex = Example('ex')
if ex:
  print(ex.get_name())

The if ex statement is evaluating to False and I am not getting any output. What is causing this instance to evaluate to False?

Upvotes: 2

Views: 727

Answers (1)

willeM_ Van Onsem
willeM_ Van Onsem

Reputation: 477794

You inherit from dict. The truthiness of a dictionary (like almost every collection) depends on whether it contains elements. This holds for lists, sets, dictionaries and strings (a string is sometimes seen as an ordered collection of characters). Furthermore a lot of libraries have collections that follow the same rules (like counters, etc.).

You construct a dictionary here that does not contain any elements. So the truthiness is False.

You can however override the truthiness again, with:

class Example(dict):
      def __init__(self, name):
        self.name = name

      def __bool__(self):
          return True

      def get_name(self):
        return self.name

Note that whether you find the truthiness of such Example(..) with an underlying empty dictionary is a decision you have to make yourself (and of course it is better to follow "Python's guidelines). So the above code fragment is not per se correct for your application. It depends on what you want to do with it.

Furthermore it can be useful to pass additional unnamed arguments (*args) and named arguments (**kwargs) to the dictionary constructor to add elements to the underlying dictionary immediately as well as modifying the __repr__ and/or __str__ functions.

class Example(dict):
    def __init__(self, name,*args,**kwargs):
        super().__init__(*args,**kwargs)
        self.name = name

    def __bool__(self):
        return True

    def get_name(self):
        return self.name

Upvotes: 7

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