Why does “return list.sort()” return None, not the list?

| | | |

👻 Check our latest review to choose the best laptop for Machine Learning engineers and Deep learning tasks!

I"ve been able to verify that the findUniqueWords does result in a sorted list. However, it does not return the list. Why?

def findUniqueWords(theList):
    newList = []
    words = []

    # Read a line at a time
    for item in theList:

        # Remove any punctuation from the line
        cleaned = cleanUp(item)

        # Split the line into separate words
        words = cleaned.split()

        # Evaluate each word
        for word in words:

            # Count each unique word
            if word not in newList:
                newList.append(word)

    answer = newList.sort()
    return answer

👻 Read also: what is the best laptop for engineering students?

Why does "return list.sort()" return None, not the list? find: Questions

Finding the index of an item in a list

5 answers

Given a list ["foo", "bar", "baz"] and an item in the list "bar", how do I get its index (1) in Python?

3740

Answer #1

>>> ["foo", "bar", "baz"].index("bar")
1

Reference: Data Structures > More on Lists

Caveats follow

Note that while this is perhaps the cleanest way to answer the question as asked, index is a rather weak component of the list API, and I can"t remember the last time I used it in anger. It"s been pointed out to me in the comments that because this answer is heavily referenced, it should be made more complete. Some caveats about list.index follow. It is probably worth initially taking a look at the documentation for it:

list.index(x[, start[, end]])

Return zero-based index in the list of the first item whose value is equal to x. Raises a ValueError if there is no such item.

The optional arguments start and end are interpreted as in the slice notation and are used to limit the search to a particular subsequence of the list. The returned index is computed relative to the beginning of the full sequence rather than the start argument.

Linear time-complexity in list length

An index call checks every element of the list in order, until it finds a match. If your list is long, and you don"t know roughly where in the list it occurs, this search could become a bottleneck. In that case, you should consider a different data structure. Note that if you know roughly where to find the match, you can give index a hint. For instance, in this snippet, l.index(999_999, 999_990, 1_000_000) is roughly five orders of magnitude faster than straight l.index(999_999), because the former only has to search 10 entries, while the latter searches a million:

>>> import timeit
>>> timeit.timeit("l.index(999_999)", setup="l = list(range(0, 1_000_000))", number=1000)
9.356267921015387
>>> timeit.timeit("l.index(999_999, 999_990, 1_000_000)", setup="l = list(range(0, 1_000_000))", number=1000)
0.0004404920036904514
 

Only returns the index of the first match to its argument

A call to index searches through the list in order until it finds a match, and stops there. If you expect to need indices of more matches, you should use a list comprehension, or generator expression.

>>> [1, 1].index(1)
0
>>> [i for i, e in enumerate([1, 2, 1]) if e == 1]
[0, 2]
>>> g = (i for i, e in enumerate([1, 2, 1]) if e == 1)
>>> next(g)
0
>>> next(g)
2

Most places where I once would have used index, I now use a list comprehension or generator expression because they"re more generalizable. So if you"re considering reaching for index, take a look at these excellent Python features.

Throws if element not present in list

A call to index results in a ValueError if the item"s not present.

>>> [1, 1].index(2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: 2 is not in list

If the item might not be present in the list, you should either

  1. Check for it first with item in my_list (clean, readable approach), or
  2. Wrap the index call in a try/except block which catches ValueError (probably faster, at least when the list to search is long, and the item is usually present.)

3740

Answer #2

One thing that is really helpful in learning Python is to use the interactive help function:

>>> help(["foo", "bar", "baz"])
Help on list object:

class list(object)
 ...

 |
 |  index(...)
 |      L.index(value, [start, [stop]]) -> integer -- return first index of value
 |

which will often lead you to the method you are looking for.

3740

Answer #3

The majority of answers explain how to find a single index, but their methods do not return multiple indexes if the item is in the list multiple times. Use enumerate():

for i, j in enumerate(["foo", "bar", "baz"]):
    if j == "bar":
        print(i)

The index() function only returns the first occurrence, while enumerate() returns all occurrences.

As a list comprehension:

[i for i, j in enumerate(["foo", "bar", "baz"]) if j == "bar"]

Here"s also another small solution with itertools.count() (which is pretty much the same approach as enumerate):

from itertools import izip as zip, count # izip for maximum efficiency
[i for i, j in zip(count(), ["foo", "bar", "baz"]) if j == "bar"]

This is more efficient for larger lists than using enumerate():

$ python -m timeit -s "from itertools import izip as zip, count" "[i for i, j in zip(count(), ["foo", "bar", "baz"]*500) if j == "bar"]"
10000 loops, best of 3: 174 usec per loop
$ python -m timeit "[i for i, j in enumerate(["foo", "bar", "baz"]*500) if j == "bar"]"
10000 loops, best of 3: 196 usec per loop

Best way to strip punctuation from a string

3 answers

Lawrence Johnston By Lawrence Johnston

It seems like there should be a simpler way than:

import string
s = "string. With. Punctuation?" # Sample string 
out = s.translate(string.maketrans("";""), string.punctuation)

Is there?

741

Answer #1

From an efficiency perspective, you"re not going to beat

s.translate(None, string.punctuation)

For higher versions of Python use the following code:

s.translate(str.maketrans("", "", string.punctuation))

It"s performing raw string operations in C with a lookup table - there"s not much that will beat that but writing your own C code.

If speed isn"t a worry, another option though is:

exclude = set(string.punctuation)
s = "".join(ch for ch in s if ch not in exclude)

This is faster than s.replace with each char, but won"t perform as well as non-pure python approaches such as regexes or string.translate, as you can see from the below timings. For this type of problem, doing it at as low a level as possible pays off.

Timing code:

import re, string, timeit

s = "string. With. Punctuation"
exclude = set(string.punctuation)
table = string.maketrans("";"")
regex = re.compile("[%s]" % re.escape(string.punctuation))

def test_set(s):
    return "".join(ch for ch in s if ch not in exclude)

def test_re(s):  # From Vinko"s solution, with fix.
    return regex.sub("", s)

def test_trans(s):
    return s.translate(table, string.punctuation)

def test_repl(s):  # From S.Lott"s solution
    for c in string.punctuation:
        s=s.replace(c,"")
    return s

print "sets      :",timeit.Timer("f(s)", "from __main__ import s,test_set as f").timeit(1000000)
print "regex     :",timeit.Timer("f(s)", "from __main__ import s,test_re as f").timeit(1000000)
print "translate :",timeit.Timer("f(s)", "from __main__ import s,test_trans as f").timeit(1000000)
print "replace   :",timeit.Timer("f(s)", "from __main__ import s,test_repl as f").timeit(1000000)

This gives the following results:

sets      : 19.8566138744
regex     : 6.86155414581
translate : 2.12455511093
replace   : 28.4436721802

Remove all special characters, punctuation and spaces from string

3 answers

I need to remove all special characters, punctuation and spaces from a string so that I only have letters and numbers.

315

Answer #1

This can be done without regex:

>>> string = "Special $#! characters   spaces 888323"
>>> "".join(e for e in string if e.isalnum())
"Specialcharactersspaces888323"

You can use str.isalnum:

S.isalnum() -> bool

Return True if all characters in S are alphanumeric
and there is at least one character in S, False otherwise.

If you insist on using regex, other solutions will do fine. However note that if it can be done without using a regular expression, that"s the best way to go about it.

315

Answer #2

Here is a regex to match a string of characters that are not a letters or numbers:

[^A-Za-z0-9]+

Here is the Python command to do a regex substitution:

re.sub("[^A-Za-z0-9]+", "", mystring)

We hope this article has helped you to resolve the problem. Apart from Why does “return list.sort()” return None, not the list?, check other find-related topics.

Want to excel in Python? See our review of the best Python online courses 2022. If you are interested in Data Science, check also how to learn programming in R.

By the way, this material is also available in other languages:



Julia Wu

Shanghai | 2022-12-02

Thanks for explaining! I was stuck with Why does “return list.sort()” return None, not the list? for some hours, finally got it done 🤗. Will get back tomorrow with feedback

Frank Richtgofen

Texas | 2022-12-02

Simply put and clear. Thank you for sharing. Why does “return list.sort()” return None, not the list? and other issues with split was always my weak point 😁. Will get back tomorrow with feedback

Walter Nickolson

Milan | 2022-12-02

split is always a bit confusing 😭 Why does “return list.sort()” return None, not the list? is not the only problem I encountered. I just hope that will not emerge anymore

Shop

Learn programming in R: courses

$

Best Python online courses for 2022

$

Best laptop for Fortnite

$

Best laptop for Excel

$

Best laptop for Solidworks

$

Best laptop for Roblox

$

Best computer for crypto mining

$

Best laptop for Sims 4

$

Latest questions

NUMPYNUMPY

Common xlabel/ylabel for matplotlib subplots

12 answers

NUMPYNUMPY

How to specify multiple return types using type-hints

12 answers

NUMPYNUMPY

Why do I get "Pickle - EOFError: Ran out of input" reading an empty file?

12 answers

NUMPYNUMPY

Flake8: Ignore specific warning for entire file

12 answers

NUMPYNUMPY

glob exclude pattern

12 answers

NUMPYNUMPY

How to avoid HTTP error 429 (Too Many Requests) python

12 answers

NUMPYNUMPY

Python CSV error: line contains NULL byte

12 answers

NUMPYNUMPY

csv.Error: iterator should return strings, not bytes

12 answers

News


Wiki

Python | How to copy data from one Excel sheet to another

Common xlabel/ylabel for matplotlib subplots

Check if one list is a subset of another in Python

sin

How to specify multiple return types using type-hints

exp

Printing words vertically in Python

exp

Python Extract words from a given string

Cyclic redundancy check in Python

Finding mean, median, mode in Python without libraries

cos

Python add suffix / add prefix to strings in a list

Why do I get "Pickle - EOFError: Ran out of input" reading an empty file?

Python - Move item to the end of the list

Python - Print list vertically