Question

I’d like to do something like:

def get_foo():   
  return self._foo or self._foo = Bar()

I am looking for the cleanest way to do it. Is it possible with or equals?

My attempts failed:

>>> foo = None
>>> foo or 'bar'
'bar'
>>> foo
>>> foo or foo = 'bar'
  File "<stdin>", line 1
SyntaxError: can't assign to operator
>>> foo or (foo = 'bar')
  File "<stdin>", line 1
    foo or (foo = 'bar')
                ^
SyntaxError: invalid syntax

Answer 1

In Python, you cannot use assignments in expressions. So you have to do the assignment and the return statement on two different lines:

def get_foo(self):
    self._foo = self._foo or Bar()
    return self._foo

But in general, it’s better to write it out:

def get_foo(self):
    if not self._foo:
        self._foo = Bar()
    return self._foo