Adding two items at a time in a list comprehension

Question

I want to add two items at a time to a list comprehension. One item is constant. How can this be achieved using only one for loop inside of a list comprehension, and no additional functions. Answers that do not use any import will be favored.

Take a look at at the following:

>>> mystring = 'ABCELKJSDLHFWEHSJDHFKHIUEHFSDF'
>>> sum([['^', char] for char in mystring.lower()], [])
['^', 'a', '^', 'b', '^', 'c', '^', 'e', '^', 'l', '^', 'k', '^', 'j', '^', 's', '^', 'd', '^', 'l', '^', 'h', '^', 'f', '^', 'w', '^', 'e', '^', 'h', '^', 's', '^', 'j', '^', 'd', '^', 'h', '^', 'f', '^', 'k', '^', 'h', '^', 'i', '^', 'u', '^', 'e', '^', 'h', '^', 'f', '^', 's', '^', 'd', '^', 'f']

I am trying to make a list with the character ^ prepended before each letter in lower case. In this example, you need to use sum to flatten the list. However, my question is, if it is possible to make a flat list in the first place. The output above is the desired output.

As in, append something constantly before a variable that changes with each iteration of the for loop. One cannot use two for loops here, as that would be too simple, for example:

mystring = 'ABCELKJSDLHFWEHSJDHFKHIUEHFSDF'
print [item for x in mystring.lower() for item in ['^', x]]

If one does something like this:

>>> mystring = 'ABCELKJSDLHFWEHSJDHFKHIUEHFSDF'
>>> [['^', x] for x in mystring]
['^', 'a', '^', 'b', '^', 'c', '^', 'e', '^', 'l', '^', 'k', '^', 'j', '^', 's', '^', 'd', '^', 'l', '^', 'h', '^', 'f', '^', 'w', '^', 'e', '^', 'h', '^', 's', '^', 'j', '^', 'd', '^', 'h', '^', 'f', '^', 'k', '^', 'h', '^', 'i', '^', 'u', '^', 'e', '^', 'h', '^', 'f', '^', 's', '^', 'd', '^', 'f']

You get lists within lists. Thus, is there a way that you can append two items at a time in a list comprehension without have to use an addition for loop or an additional function like sum? I ask this, because its something quite simple, yet I can't find a way to do it. If one tries to do the following:

>>> ['^', x for x in mystring.lower()]
  File "<console>", line 1
    ['^', x for x in mystring.lower()]
              ^
SyntaxError: invalid syntax

The attempt gives a SyntaxError. So, is what I'm asking impossible to do in Python? Using () gives me a list of tuples.

I've also tried using the splat/unpacking operator:

>>> [*['^', x] for x in mystring.lower()]
  File "<console>", line 1
    [*['^', x] for x in mystring.lower()]
     ^
SyntaxError: invalid syntax

But as above, this too is a syntax error. Sorry for this late edit, but I have tried the following:

import itertools
mystring = "HELLOWORLD"
print(list(itertools.chain.from_iterable(('^', x) for x in mystring.lower())))

But the above still required an import.

Answer 1

You can use itertools.chain.from_iterable, this is equivalent to that nested list comprehension version but slightly efficient(for large lists):

>>> from itertools import chain
>>> mystring = 'ABCELKJSDLHFWEHSJDHFKHIUEHFSDF'
>>> list(chain.from_iterable([['^', x] for x in mystring]))
['^', 'A', '^', 'B', '^', 'C', '^', 'E', '^', 'L', '^', 'K', '^', 'J', '^', 'S', '^', 'D', '^', 'L', '^', 'H', '^', 'F', '^', 'W', '^', 'E', '^', 'H', '^', 'S', '^', 'J', '^', 'D', '^', 'H', '^', 'F', '^', 'K', '^', 'H', '^', 'I', '^', 'U', '^', 'E', '^', 'H', '^', 'F', '^', 'S', '^', 'D', '^', 'F']

In Python 3.3+ you can also use yield from in a generator function:

>>> mystring = 'ABCELKJSDLHFWEHSJDHFKHIUEHFSDF'
>>> def solve(strs):
...     for x in strs:
...         yield from ['^', x]
...         
>>> list(solve(mystring))
['^', 'A', '^', 'B', '^', 'C', '^', 'E', '^', 'L', '^', 'K', '^', 'J', '^', 'S', '^', 'D', '^', 'L', '^', 'H', '^', 'F', '^', 'W', '^', 'E', '^', 'H', '^', 'S', '^', 'J', '^', 'D', '^', 'H', '^', 'F', '^', 'K', '^', 'H', '^', 'I', '^', 'U', '^', 'E', '^', 'H', '^', 'F', '^', 'S', '^', 'D', '^', 'F']

Answer 2

You can start with this:

print list( '^'.join(mystring.lower()) )

which gives:

['a', '^', 'b', '^', 'c', '^', ...]

So this would give the desired output:

l = list( '^'.join(mystring.lower()) )
l.insert(0, '^')
print l

And another way:

print [ y for x in zip(['^'] * len(mystring), mystring.lower()) for y in x ]

which gives:

['^', 'a', '^', 'b', '^', 'c', ...