Friday, February 24, 2017

Perl-like "unless" (reverse if) feature in Python

By Vasudev Ram

Flowchart image attribution

I was mentally reviewing some topics to discuss for a Python training program I was running. Among the topics were statements, including the if statement. I recollected that some languages I knew of, such as Perl, have an unless statement, which is like a reverse if statement, in that only the first nested suite (of statements) is executed if the Boolean condition is false, whereas only the second nested suite is executed if the condition is true. See the examples below.

The term "suite" used above, follows the terminology used in Python documentation such as the Python Language Reference; see this if statement definition, for example.

That is, for the if statement:
if condition:
    suite1  # nested suite 1
    suite2  # nested suite 2
results in suite1 being run if condition is true, and suite2 being run if condition is false, whereas, for the unless statement:
unless condition:
, the reverse holds true.

Of course, there is no unless statement in Python. So I got the idea of simulating it, at least partially, with a function, just for fun and as an experiment. Here is the first version, in file
# v1

# A simple program to partially simulate the unless statement 
# (a sort of reverse if statement) available in languages like Perl.
# The simulation is done by a function, not by a statement.

# Author: Vasudev Ram
# Web site:
# Blog:
# Product store:

# Define the unless function.
def unless(cond, func_if_false, func_if_true):
    if not cond:

# Test it.
def foo(): print "this is foo"
def bar(): print "this is bar"

a = 1
# Read the call below as:
# Unless a % 2 == 0, call foo, else call bar
unless (a % 2 == 0, foo, bar)
# Results in foo() being called.

a = 2
# Read the call below as:
# Unless a % 2 == 0, call foo, else call bar
unless (a % 2 == 0, foo, bar)
# Results in bar() being called.
Here is the output:
$ python
this is foo
this is bar
This simulation of unless works because functions are objects in Python (since almost everything is an object in Python, like almost everything in Unix is a file), so functions can be passed to other functions as arguments (by passing just their names, without following the names with parentheses).

Then, inside the unless function, when you apply the parentheses to those two function names, they get called.

This approach to simulation of the unless statement has some limitations, of course. One is that you cannot pass arguments to the functions [1]. (You could still make them do different things on different calls by using global variables (not good), reading from files, or reading from a database, so that their inputs could vary on each call).

[1] You can actually pass arguments to the functions in a few ways, such as using the *args and **kwargs features of Python, as additional arguments to unless() and then forwarding those arguments to the func_if_false() and func_if_true() calls inside unless().

Another limitation is that this simulation does not support the elif clause.

However, none of the above limitations matter, of course, because you can also get the effect of the unless statement (i.e. a reverse if) by just negating the Boolean condition (with the not operator) of an if statement. As I said, I just tried this for fun.

The image at the top of the post is of a flowchart.

For something on similar lines (i.e. simulating a language feature with some other code), but for the C switch statement simulated (partially) in Python, see this post I wrote a few months ago:

Simulating the C switch statement in Python

And speaking of Python language features, etc., here is a podcast interview with Guido van Rossum (creator of the Python language), about the past, present and future of Python.


- Vasudev Ram - Online Python training and consulting

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