Creating stripped nested lists of strings using list comprehension

Question

I'm performing an exercise on CodeCademy revolving string manipulation, and I tried performing the task using list comprehension but couldn't manage it correctly: the correct way to perform it is to have a nested list, each consisting of the stripped strings. Here's how I managed to do it correctly:

transactions_clean = []
for transaction in daily_transactions_split:
  new_trans = []
  for i in range(len(transaction)):
    new_trans.append(transaction[i].strip('\n').strip())
  transactions_clean.append(new_trans)

This yields the result:

[['Edith Mcbride', '$1.21', 'white', '09/15/17'], ['Herbert Tran', '$7.29', 'white&blue', '09/15/17']]

which is exactly what I'm looking for.

This method uses a nested loop, and I was interested in performing it using a list comprehension. I managed to come up with:

transactions_clean = [st.strip('\n').strip() for transaction in daily_transactions_split for st in transaction]

which manages to strip the strings correctly, but creates a non-nested list of strings. Could my goal be achieved using comprehensions?

Thanks

Robin Zigmond · Accepted Answer · 2018-12-02 13:37:42Z

1

You're not getting a nested list because you only have one pair of [] in your list comprehenstion (and the expression you have there doesn't produce a list itself). Just do this instead (note that I've had to swap the order of your for clauses as well as insert the inner []s):

transactions_clean = [[st.strip('\n').strip() for st in transaction] for transaction in daily_transactions_split]

answered Dec 2, 2018 at 13:37

Robin Zigmond

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1 Comment

Barker Over a year ago

It works. I tried going for the extra pair of [ ] earlier but it was of no avail, I did not flip the order of my for clauses. Thanks man

Community · Accepted Answer · 2020-06-20 09:12:55Z

The pattern [(x, y) for x in xs for y in ys] is the equivalent of itertools.product(xs, ys).

The comprehension you are writing doesn't require a product of two collections.

import itertools

items = [(x, y) for x, y in itertools.product(range(5), range(10, 15))]
print(len(items), items)

Note the new collection is 5 * 5 in length, i.e. the product of the number of items in each collection.

OUTPUT:

25 [(0, 10), (0, 11), (0, 12), (0, 13), (0, 14), (1, 10), (1, 11), (1, 12), (1, 13), (1, 14), (2, 10), (2, 11), (2, 12), (2, 13), (2, 14), (3, 10), (3, 11), (3, 12), (3, 13), (3, 14), (4, 10), (4, 11), (4, 12), (4, 13), (4, 14)]

import random
from string import whitespace

example_data =[
    ['Edith Mcbride', '$1.21', 'white', '09/15/17'],
    ['Herbert Tran', '$7.29', 'white&blue', '09/15/17'],
]

# Make the data dirty again.
transactions_dirty = [[f'{random.choice(whitespace) * random.randint(1, 4)}{item}{random.choice(whitespace) * random.randint(1, 4)}'
                       for item in transaction]
                      for transaction in example_data]
print(transactions_dirty)

# Clean the data.
transactions_clean = [[item.strip() for item in transaction]
                      for transaction in transactions_dirty]
print(transactions_clean)

OUTPUT:

[['\x0b\x0b\x0b\x0bEdith Mcbride\r\r\r', '\x0c\x0c$1.21\x0b', '\x0bwhite ', '\n\n09/15/17\t\t'], ['\t\t\tHerbert Tran\x0c\x0c\x0c', '\t\t$7.29\n\n\n', ' white&blue\r', '\n09/15/17 ']]

[['Edith Mcbride', '$1.21', 'white', '09/15/17'], ['Herbert Tran', '$7.29', 'white&blue', '09/15/17']]

Note that .strip('\n').strip() does the same thing as .strip()

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