What's Python equivalent to or equals expression, to get return foo or foo = 'bar' working?

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

Python's assignment operator doesn't pass through the assigned value, primarily because in other languages it has proven to be a fruitful generator of accidental =/== bugs. Instead you have to be explicit:

def get_foo(self):
    if self._foo is None:
        self._foo= Bar()
    return self._foo

You can make an expression with side-effects if you really want to:

def set_foo(self, v):
    self._foo= v
    return v

return self._foo or self.set_foo(Bar())

but it's generally considered a Bad Thing.

Answer 2

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

Answer 3

I might be wrong, but this looks like a job for the ||= operator.

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

My Perl is rusty at best, but ||= seems to return an R-value, so the return should work.

Edit: Woopsie, this is Python. It doesn't have a ||=. Bummer.

Then I'd use the simulated ternary operator:

def get_foo()
  self._foo = self._foo is None and Bar() or self._foo
  return self._foo

Edit 2: Python has a real ternary operator. Use it instead of the above.

def get_foo()
  self._foo = Bar() if self._foo is None else self._foo
  return self._foo