Python equivalent of setting if not null

问题

In ruby I can do this:

1.9.3-p448 :001 > a = 1 || 2
 => 1 
1.9.3-p448 :004 > a = nil || 2
 => 2 
1.9.3-p448 :005 > a = 1 || nil
 => 1 

Is there a similar one-liner in Python?

回答1:

Just use the or operator. From the linked page:

x or y: if x is false, then y, else x

Example:

In [1]: 1 or 2
Out[1]: 1

In [2]: None or 2
Out[2]: 2

In [3]: 1 or None
Out[3]: 1

回答2:

Python's or operator is pretty much the equivalent of Ruby's || -- and None can be used in Python somewhat similarly to how nil is in Ruby.

So, for example,

a = None or 2

would set a to 2.

You can also use a richer "ternary" operator, something if condition else somethingelse -- a or b is the same as a if a else b -- but clearly or is more concise and readable when what you want to do is exactly the semantics it supports.

回答3:

Don't forget about modern if-else syntax:

x = a if a is not None else 999

(or whatever condition you need). This let you test for non-None and is not prone to empty list and similar problems.

General syntax is

ValueToBeUsedIfConditionIsTrue if Condition else ValueToBeUsedIfConditionIsFalse

来源：https://stackoverflow.com/questions/28308648/python-equivalent-of-setting-if-not-null