I have list of values:
md5 = ['1111', '3333', '44444', '555555', '56632423', '23423514', '2342352323']
I want to concatenate them into a Query String:
'md5=1111&md5=3333&md5=44444&md5=555555&md5=56632423&md5=23423514&md5=2342352323'
What is the best way to do that?
Since you're building a query string, it's better to use a function from the standard library that's specifically designed to do so:
urllib.parse.urlencode() (in Python 3)urllib.urlencode() (in Python 2)... than to muck around with str.join(). Here's how you'd use it:
from urllib.parse import urlencode # Python 3
# from urllib import urlencode # Python 2
md5 = ['1111', '3333', '44444', '555555', '56632423', '23423514', '2342352323']
urlencode([("md5", x) for x in md5])
Result:
'md5=1111&md5=3333&md5=44444&md5=555555&md5=56632423&md5=23423514&md5=2342352323'
Alternatively (thanks to Jon Clements in the Python chatroom), you can pass a dictionary to urlencode and the parameter doseq=True:
urlencode({'md5': md5}, doseq=True)
... which produces the same result. This is explained in the documentation linked above:
The value element in itself can be a sequence and in that case, if the optional parameter doseq is evaluates to True, individual
key=valuepairs separated by'&'are generated for each element of the value sequence for the key. The order of parameters in the encoded string will match the order of parameter tuples in the sequence.
urllib.parse.urlencode instead of urllib.urlencode in Python 3.x.md5s = '&'.join('md5='+m for m in md5)
The part inside the parentheses is a generator expression, which gives you 'md5='+m for each m in the original list.
The join function takes that expression and links it into one big string, putting an & between each pair.
You can use join():
md5=['1111','3333','44444','555555','56632423','23423514','2342352323']
print 'md5=' + '&md5='.join(md5)
join for the part that joins up elements, everything makes sense. If you try to use it for additional string processing, you end up with fencepost problems where you need to add an extra md5= at the beginning (your answer) or remove a & at the end (ljlozano's answer), which makes it a lot easier to get something wrong.