From the perspective of a network engineer who has just started out on their journey to learn python and network automation, one of the first intimidating concepts that they are likely to come across is the list comprehensions. In this post we will try and cover the basics of python list comprehensions
If you have overcome the hurdle of understanding python’s conditional statements like for and if/else, the next step is to make the code concise at the cost of readability at first but gradually once you get hang of comprehensions, the readability isn’t that big of an issue.
Let’s take an example to take a look at the most common list comprehension generic patterns.
For loop containing single IF statement
devices = ['xxx-indi-ce01', 'xxx-indi-ce02', 'xxx-usam-ce01', 'xxx-usam-ce02', 'xxx-cana-ce01', 'xxx-cana-ce02', '', '', None]
device_list = []
for device in devices:
if device:
device_list.append(device)# List comprehension equivalent of above
device_list = [device for device in device_list if device]
# Generic syntax
device_list = [append value in list for each value in list ifcondition=True]
For loops containing IF/ELSE Statements
devices = ['xxx-indi-ce01', 'xxx-indi-ce02', 'xxx-usam-ce01', 'xxx-usam-ce02', 'xxx-cana-ce01', 'xxx-cana-ce02', '', '', None]
device_list = []
for device in devices:
if device:
device_list.append(device)
else:
device_list.append('PLACEHOLDER')
print(device_list)
['xxx-indi-ce01', 'xxx-indi-ce02', 'xxx-usam-ce01', 'xxx-usam-ce02', 'xxx-cana-ce01', 'xxx-cana-ce02', 'PLACEHOLDER', 'PLACEHOLDER', 'PLACEHOLDER']# List comprehension equivalent of above
device_list = [device if device else "PLACEHOLDER" for device in devices]
# Generic syntax
device_list = [append value ifcondition=True else append else block value for device in list]
For loops containing IF/ELIF/ELSE Statements
This one is a little tricky and there is an increased loss of readability. There is no elif keyword usage while using list comprehension, you could only use if and else. So the workaround is to break down elif into if and else.
devices = ['xxx-indi-ce01', 'xxx-indi-ce02', 'xxx-usam-ce01', 'xxx-usam-ce02', 'xxx-cana-ce01', 'xxx-cana-ce02', '', '', None]
device_list = []
for device in devices:
if device:
device_list.append(device)
elif device == '':
device_list.append('EMPTY')
else:
device_list.append('PLACEHOLDER')
print(device_list)
['xxx-indi-ce01', 'xxx-indi-ce02', 'xxx-usam-ce01', 'xxx-usam-ce02', 'xxx-cana-ce01', 'xxx-cana-ce02', 'EMPTY', 'EMPTY', 'PLACEHOLDER']# List comprehension equivalent of above
device_list = [device if device else "EMPTY" if device == '' else "PLACEHOLDER" for device in devices]
# Generic syntax
device_list = [append value ifcondition=True else append '' if elif condition is True else append 'else value' for each value in list]The list comprehension statement above will make much more sense if we breakdown elif into if and else like below
device_list = []
for device in devices:
if device:
device_list.append(device)
else:
if device == '':
device_list.append('EMPTY')
else:
device_list.append('PLACEHOLDER')
print(device_list)
Multiple nested IF’s
device_list = []
for device in devices:
if device:
if 'indi' in device:
device_list.append(device)
print(device_list)
['xxx-indi-ce01', 'xxx-indi-ce02']device_list = [device for device in devices if device if 'indi' in device]If you notice carefully, the syntax of list comprehension has if else statement ahead of “for device in devices” if we are dealing with if and else together. However if our reference code has only if statements, the if statements are followed after the “for device in devices” in comprehension. Following the same rule, multiple if statements are also handled after “for device in devices” and are just stacked together one after the other in sequence.
Above comprehension could also have been written as
device_list = [device for device in devices if device and 'indi' in device]
which is equivalent to
device_list = []
for device in devices:
if device and 'indi' in device:
device_list.append(device)Conclusion:-
While list comprehensions make your code look beautiful it can come at the cost of lack of readability. As per Zen of python, clear is better than concise. Comprehension is something that one should use gradually and build upon as you use it more and more often. Secondly, after understanding the basic rules of comprehension, write complex comprehensions become relatively easier if you can breakdown the conventional for loop with nested if and else blocks into simpler if else heirarchy.
