[Tutor] Pythonic way of concatenation of elements in an array

Steven D'Aprano steve at pearwood.info
Fri Jan 27 00:04:18 CET 2012

spawgi at gmail.com wrote:
> Hello,
> My code is -
> l = len(m)
> item = str(m[1])
> for i in range(2,l):
>     item = item + "-" + str(m[i])
> This code is part of a bigger function. It works fine. But I am not happy
> with the way I have written it. I think there is a better (Pythonic) way to
> rewrite it.
> If anyone knows how to improve this snippet, I would be very thankful.

(1) Never use "l" as a variable name, as it looks too much like 1.

(2) Use meaningful names. When posting snippets for help, you should show 
example data.

(3) You almost never need to iterate over an index by hand. Instead iterate 
over the items themselves. That is, instead of this:

for i in len(sequence):
     do_something_with( sequence[i] )

do this instead:

for item in sequence:
     do_something_with( item )

and similar variations.

(4) When you want only part of a sequence, use slicing to extract just the 
parts you care about.

(5) When assembling strings from substrings, never use repeated concatenation 
using + as that can be EXTREMELY slow. Use str.join to build the string in one 
assignment, instead of multiple assignments.

Your code shown above is *very* inefficient and will be PAINFULLY slow if m is 
very large. To understand why, you should read this article:


In this case, you can replace your snippet with this:

result = '-'.join(str(item) for item in m[1:])

For example:

py> m = [1, 2, "hello world", (23, 42), 5]
py> '-'.join(str(item) for item in m[1:])
'2-hello world-(23, 42)-5'


More information about the Tutor mailing list