Python's equivalent of Ruby's ||=

Question

To check if a variable exist, and if exits, use the original value, other wise, use the new value assigned. In ruby, it's var ||= var_new

How to write it in python?

PS: I don't know the name of ||=, I simply can't search it in Bing.

Answer 1

I think there is some confusion from the people who aren't really sure what the conditional assignment operator (||=) does, and also some misunderstanding about how variables are spawned in Ruby.

Everyone should read this article on the subject. A TLDR quote:

A common misconception is that a ||= b is equivalent to a = a || b, but it behaves like a || a = b

In a = a || b, a is set to something by the statement on every run, whereas with a || a = b, a is only set if a is logically false (i.e. if it's nil or false) because || is 'short circuiting'. That is, if the left hand side of the || comparison is true, there's no need to check the right hand side.

And another very important note:

...a variable assignment, even if not run, immediately summons that variable into being.

# Ruby
x = 10 if 2 == 5
puts x

Even though the first line won't be run, x will exist on the second line and no exception will be raised.

This means that Ruby will absolutely ensure that there is a variable container for a value to be placed into before any righthand conditionals take place. ||= doesn't assign if a is not defined, it assigns if a is falsy (again, false or nil - nil being the default nothingness value in Ruby), whilst guaranteeing a is defined.

What does this mean for Python?

Well, if a is defined, the following:

# Ruby
a ||= 10

is actually equivalent to:

# Python
if not a:
    a = 10

while the following:

# Either language
a = a or 10

is close, but it always assigns a value, whereas the previous examples do not.

And if a is not defined the whole operation is closer to:

# Python
a = None
if not a:
    a = 10

Because a very explicit example of what a ||= 10 does when a is not defined would be:

# Ruby
if not defined? a
    a = nil
end

if not a
    a = 10
end

At the end of the day, the ||= operator is not completely translatable to Python in any kind of 'Pythonic' way, because of how it relies on the underlying variable spawning in Ruby.