# [Tutor] manipulating lists

Dave Angel davea at ieee.org
Wed Jul 29 21:29:17 CEST 2009

```(You omitted a title, so I made one up.  Hope it's okay)

Chris Castillo wrote:
> # Module demonstrates use of lists and set theory principles
>
> def Unite(set1, set2):		# evaluate 2 lists, join both into 1 new list
> 	newList = []
> 	for item in set1:
> 		newList.append(item)
> 	for item in set2:
> 		newList.append(item)
> 	newList.sort()
> 	return newList
>
> def Intersect(set1, set2):		# evaluate 2 lists, check for
> commonalities, output commonalities to 1 new list
> 	newList = []
> 	for item in set1:
> 		if item in set1 and item in set2:
> 			newList.append(item)
> 	newList.sort()
> 	return newList
>
> def Negate(set1, set2):		# evaluate 2 lists, return negation of 1st list
> 	newList = []
> 	for item in set1:
> 		if item in set2:
> 			set1.remove(item)
> 	newList = set1
> 	return newList
>
>
> could this be done in a more elegant fashion?
>
>
Note:  don't ever use tabs in source code.  Use spaces to indent -
preferably 4 each.  If you accidentally mix spaces and tabs, you could
have very mysterious bugs

Just by inspection - untested code:

#Use docstring.  Use extend rather than append in loop.  Use slice
notation to make a copy of the items in the list

def Unite(set1, set2):
"""  evaluate 2 lists, join both into 1 new list"""
newList = set1[:]
newList.extend(set2)
return sorted(newList)

def Intersect(set1, set2):
""" evaluate 2 lists, check for
commonalities, output commonalities to 1 new list"""
newList = []
for item in set1:
if item in set2:
newList.append(item)
return sorted(newList)

#This one was buggy;  it modified set1 in place, and returned another reference to it
def Negate(set1, set2):
""" evaluate 2 lists, items that are in first list, but not in second """
newList = set1[:]
for item in set2:
newList.remove(item)
return newList         #Question:  did you mean to sort this one too?

Now, could these be improved further?  Absolutely.  Using list comprehensions or similar techniques, some of these loops could be one-liners.
But I'm guessing you're a beginner in Python, and it's more important that you get used to writing readable, robust code than clever code.

One thing I do recommend, even at this stage, is to study set().

DaveA

```

More information about the Tutor mailing list