Python/ThreePartOperator - Revision history

Griswolf: minor correction

2011-10-05T18:55:01Z

minor correction

Griswolf: better example using a False default: It still works

2011-10-05T18:53:57Z

better example using a False default: It still works

Griswolf: rewrite to mention conditional operator

2011-10-05T18:52:18Z

rewrite to mention conditional operator

Griswolf at 18:34, 5 October 2011

2011-10-05T18:34:26Z

Griswolf: add final note

2011-10-05T18:33:42Z

add final note

Griswolf: add example of default assignment

2011-10-05T18:30:02Z

add example of default assignment

New page

Python does not supply the ?: operator, but it is sometimes very nice. Here's how to mimic it

We often want to set local variables to "what was passed, or if not, a default" like this:
<syntaxhighlight lang="python">
def example(required, alist=None, anum=None, astring=''):
my_list = alist or ['item']
my_num = anum or 666
my_string = astring or 'string'
print('required: "%s"\nlist: %s\nNumber: %s\nString: "%s"'%(required, my_list,my_num,my_string))

example('must') # prints 'must' plus the three defaults
example('yes',anum=99) # prints 'yes', default, 99, default
# etc
</syntaxhighlight>.

@@ Line 9: / Line 9: @@
    print('required: "%s"\nlist: %s\nNumber: %s\nString: "%s"'%(required, my_list,my_num,my_string))
 </syntaxhighlight>
-Since version 2.5, however, Python has an official  [http://docs.python.org/reference/expressions.html#conditional-expressions ternary operator] called a "conditional operator": <code>x if C else y</code> evaluates C first, then either x or y.
+Since version 2.5, however, Python has an official  [http://docs.python.org/reference/expressions.html#conditional-expressions ternary operator] called a "conditional expression": <code>x if C else y</code> evaluates C first, then either x or y.

@@ Line 13: / Line 13: @@
 # etc
 </syntaxhighlight>
-Note the use of the '''or''' operator. Python uses lazy left to right evaluation of that operator, so if the left operand is "True" the assignment takes it, else it takes the right operand.
+Note the use of the '''or''' operator. Python uses lazy left to right evaluation of that operator, so if the left operand evaluates as True the assignment takes it, else it takes the right operand.
 There are some issues (what if you want the default to be something that evaluates as False?) but this technique will work often enough to be worth knowing.

@@ Line 5: / Line 5: @@
 def example(required, alist=None, anum=None, astring=''):
    my_list = alist or ['item']
-   my_num = anum or 666
+   my_num = anum or 0 # note that 0 evaluates False, but is still chosen here
    my_string = astring or 'string'
    print('required: "%s"\nlist: %s\nNumber: %s\nString: "%s"'%(required, my_list,my_num,my_string))
 </syntaxhighlight>
 Since version 2.5, however, Python has an official  [http://docs.python.org/reference/expressions.html#conditional-expressions ternary operator] called a "conditional operator": <code>x if C else y</code> evaluates C first, then either x or y.

@@ Line 12: / Line 12: @@
 example('yes',anum=99) # prints 'yes', default, 99, default
 # etc
-</syntaxhighlight>.
+</syntaxhighlight>