What is the most Pythonic way to provide a fall-back value in an assignment?

Question

In Perl, it\'s often nice to be able to assign an object, but specify some fall-back value if the variable being assigned from is \'undef\'. For instance:

my $x         


        

Answer 1

Most of the solutions relying on if statements don't work for the case where x is 0 or negative.

>>> x = 0
>>> y = 2
>>> a = x or y
>>> a
2
>>> 

If you knew the name of the variable ahead of time you could look for like so:

if 'x' in dir():
    a = x 
except:
     a =y

However that solution seems kind of sloppy to me. I believe the best method is to use a try except block like so:

try:
    a = x
else:
    a = y

Answer 2

One way to rewrite...

if x is not None
    a = x
else
    a = y

..is:

x = myfunction()

if x is None:
    x = y

print x

Or, using exceptions (possibly more Python'y, depending on the what the code is doing - if it returns None because there was an error, using an exception is probably the correct way):

try:
    x = myfunction()
except AnException:
    x = "fallback"

print x

All that said, there really isn't anything wrong with you original code:

if x is not None
    a = x
else
    a = y

It's long, but I find that far easier to read (and much more Pythonic) than either of the following one-liners:

a = x if x is not None else y
a = x or y

Answer 3

first you can do your full-on with a ternary:

a = y if x is None else x

but it doesn't solve your problem. what you want to do is more closely implemented with:

try:
    a = x
except:
    a = y

Answer 4

There's python's ternary operation:

a = x if x is not None else y

Available in 2.5 and up.

Answer 5

Since 2.5:

If you want to fall back only on None:

a = x if x is not None else y 

If you want to fall back also on empty string, false, 0 etc.:

a = x if x else y 

or

a = x or y 

---

As for undefined (as never defined, a.k.a. not bound):

try:
  a = x 
except NameError:
  a = y

or a bit more hackish (I'd not really recommend that, but it's short):

a = vars().get('x',y)

Answer 6

Just some nitpicking with your Perl example:

my $x = undef;

This redundant code can be shortened to:

my $x;

And the following code doesn't do what you say it does:

my $a = $x || $y;

This actually assigns $y to $a when $x is false. False values include things like undef, zero, and the empty string. To only test for definedness, you could do the following (as of Perl 5.10):

my $a = $x // $y;