List comprehension in format string? (Python)

Question

Let's say I have a list datalist, with len(datalist) = 4. Let's say I want each of the elements in the list to be represented in a string in this way:

s = "'{0}'::'{1}'::'{2}' '{3}'\n".format(datalist[0], datalist[1], datalist[2], datalist[3])

I don't like having to type datalist[index] so many times, and feel like there should be a more effective way. I tried:

s = "'{0}'::'{1}'::'{2}' '{3}'\n".format(datalist[i] for i in range(4))

But this doesn't work:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: tuple index out of range

Does anybody know a functioning way to achieve this efficiently and concisely?

juanpa.arrivillaga · Accepted Answer · 2017-08-23 20:46:22Z

6

Yes, use argument unpacking with the "splat" operator *:

>>> s = "'{0}'::'{1}'::'{2}' '{3}'\n"
>>> datalist = ['foo','bar','baz','fizz']
>>> s.format(*datalist)
"'foo'::'bar'::'baz' 'fizz'\n"
>>>

Edit

As pointed out by @AChampion you can also just use indexing inside the format string itself:

>>> "'{0[0]}'::'{0[1]}'::'{0[2]}' '{0[3]}'\n".format(datalist)
"'foo'::'bar'::'baz' 'fizz'\n"

edited Aug 23, 2017 at 20:46

answered Aug 23, 2017 at 20:35

juanpa.arrivillaga

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5 Comments

juanpa.arrivillaga Over a year ago

@Sandi yes, I've linked to the relevant documentation.

AChampion Over a year ago

You can also just index in the format string "'{0[0]}'::'{0[1]}'::'{0[2]}' '{0[3]}'\n".format(datalist)

juanpa.arrivillaga Over a year ago

@Sandi read through the answers to this question

kronosjt Over a year ago

Fot this method to work I'd have to know how many items the list has. What if I don't know beforehand how many items are in the list?

juanpa.arrivillaga Over a year ago

@kronosjt it depends on exactly what you are trying to accomplish. What sort of string are you trying to construct?

SarahWallace · Accepted Answer · 2023-10-21 22:16:47Z

0

List comprehension with enumerate() may be what you are looking for. You would not need to reference a list length with enumerate.

_>>enumerate(datalist) will return a tuple numbering the list items starting with default 0.

datalist = ['foo','bar','baz','fizz']    
[print('{}::{} '.format(index,data),end='') for index,data in enumerate(datalist)]
print()  #start new line

Output:

0::foo 1::bar 2::baz 3::fizz

answered Oct 21, 2023 at 22:16

SarahWallace

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