如何以“ pythonic”方式将单独的字符串添加到列表中的各种字符串？

Question

I have a list of strings. I need to add a string to all of them, and then a second string to only two of them. I am trying the following:

[s + ' help ' for s in [ ['me '].extend(['with ' + t for t in ['things', 'please']]) ] ]

What I would expect is the following:

['me help ', 'with things help ', 'with please help ']

Nothing happens when I try to run this in iPython. It seems to have a problem with using the extend method on a 'newly created' list. I am unsure, and relatively new to python.

EDIT:

To be more explicit, in the above example, the original list would be:

['me', 'things', 'please']

I need to add 'help ' to all of them, and 'with ' to only the latter two.

And I get an error when running the above line of code:

TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'

Second Edit:

I am more focused on the location of the elements. I want a string added to every element of the list, excluding the first, and then a second string added to the entire new list. My above approach was a first stab at what it should be (in the most 'pythonic' way), but it does not work. I understand why it does not work, but I am unsure how to rewrite the above line into something that will work.

Answer 1

In case you want to index the elements in the list you can use this:

 s=['me', 'things', 'please']
   ['with ' + x + ' help' if x!=s[0] else x+' help' for x in s]

Answer 2

Unless you define a function, which extends a list and returns the result:

def lext(l, lx):
    l.extend(lx)
    return l

(which python already does):

import itertools
lext = itertools.chain

and employ it this way:

[s + ' help ' for s in lext(['me '], ['with ' + t for t in ['things', 'please']])]

you have to do it step by step:

inp = ['me', 'things', 'please']
out1 = [ s + ' help' for s in inp]
out2 = [ out1[0] ]
out2.extend(('with ' + s for s in out1[1:]))

Welcome to the wonderful world of procedures and functions, aka. commands and expressions :)

Answer 3

If it's string match based:

['with ' + t + ' help' if t != 'me' else t + ' help' for t in ['me', 'things', 'please']]

This will create a list of strings based if an item is me or not.

Produces:

['me help', 'with things help', 'with please help']

If it's positional:

If you always know the position, this might help:

x = ['me', 'things', 'please']
word_pos = 1
['{} help'.format(i) for i in x[:word_pos]] + ['with {} help'.format(i) for i in x[word_pos:]]

Which produces:

['me help', 'with things help', 'with please help']

Either one can be a one-liner, but I'd strongly suggest expanding these as they quickly get fudgy to work with.

Answer 4

This is a possible solution to achieve exactly what you wrote you expect:

items = ['me', 'things', 'please']
new_items = []

for index, value in enumerate(items):
    # Prepend to every item but the first
    if i > 0:
        v = "with %s" % v

    # Append to every item
    v = "%s help " % v

    new_items.append(v)

print(new_items)  # outputs: ['me help ', 'with things help ', 'with please help ']

This appends help to every item, and prepends with to every but the first item.

Answer 5

I think I figured this out. Thanks all for your help, but this is (most simply) what I was trying to get at:

[s + ' help ' for s in  ['me '] + ['with ' + t for t in ['things', 'please']]   ]

which produces

['me  help ', 'with things help ', 'with please help ']

It was just the extend method that I needed to replace with + to concatenate the list.