在Python中使用Bash $ {var：-default}等价物？

Question

What's the simplest way to perform default variable substitution?

x = None
... (some process which may set x)
if x is None: use_x = "default"
else:         use_x = x

Is there any way of writing this in one line?

Answer 1

You can use a conditional expression :

use_x = "default" if x is None else x

You could use a dict defaulting to x if the key did not exist to resemble the bash syntax but the conditional would be the idiomatic way:

use_x = {None: "default"}.get(x, x)

Answer 2

You can use

x = x or "default"

in Python. This is not exactly the same, since it depends on Python definition of truth values , which differs from Bash's distinction of set and unset values. But it's a close approximation and the idiomatic Python way of solving the same general problem of testing for unset values.

If you are specifically interested in only substituting None values, then this code will yield the wrong result, and Padraic's solution is more appropriate.