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I'm rather new to python and I'm not sure how to do the following.

I have a list foo containing metadata and measurement values for a certain plot. For example, a plotID, survey date, initials of surveyors, several measurement values and a categorical variable.

foo= ['plot001', '01-01-2013', 'XX', '10', '12.5', '0.65', 'A']

Because these data are read from a .txt, all list items are strings. My question is: how do I convert each list item to the approporiate datatype?. I can make a list with the desired data types:

dType= ['str', 'str', 'str', 'int', 'float', 'float', 'float', 'str']

It would be nice if I could apply each item in dType as function to the matching element in foo as such: 

out= [str(foo[0]), str(foo[1]), str(foo[2]), int(foo[3]), float(foo[4]), float(foo[5]), float(foo[6]), str(foo[7])]

but I'm sure there must be a better solution! Thanks for any suggestions!

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  • Yes, the suggestion of @roippi works very fine. Thanks a lot! Commented Dec 14, 2013 at 19:40
  • And @John1024 also suggested this, thanks! Commented Dec 14, 2013 at 19:41

3 Answers 3

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Instead of a list of strings, make dType a list of builtin factories:

dType= [str, str, str, int, float, float, str]

(you had an extra float that I removed)

Then just use zip:

[t(x) for t,x in zip(dType,foo)] Out[6]: ['plot001', '01-01-2013', 'XX', 10, 12.5, 0.65, 'A']

Bonus: you could even get fancy and make your own factory functions and apply them in the same manner with functools.partial. Say, if you wanted that date to turn into a datetime object:

def datetime_factory(format,s): from datetime import datetime return datetime.strptime(s,format) from functools import partial dType= [str, partial(datetime_factory,'%d-%m-%Y'), str, int, float, float, str] [t(x) for t,x in zip(dType,foo)] Out[29]: ['plot001', datetime.datetime(2013, 1, 1, 0, 0), 'XX', 10, 12.5, 0.65, 'A']

(making our own factory def was needed here since partial only allows you to partially apply the leftmost arguments, and strptime requires the string-to-be-formatted first)

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foo= ['plot001', '01-01-2013', 'XX', '10', '12.5', '0.65', 'A'] fnlist= [str, str, str, int, float, float, str] out = [fn(v) for fn, v in zip(fnlist, foo)]
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ast.literal_eval could be helpful:

>>> import ast >>> def evaluate(x): ... try: ... return ast.literal_eval(x) ... except (ValueError,SyntaxError): ... return x ... >>> map(evaluate, foo) ['plot001', '01-01-2013', 'XX', 10, 12.5, 0.65, 'A']
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this doesn't work for the dates. I got ValueError: malformed string
Hi, thanks for your suggestions. Unfortunataly, it doesn't seem to work for me. The error reads: ValueError: malformed string
yeah, you're right. Hold on ... (that's what I get for answering and watching TV at the same time)
Yes, thanks, it works now! This is very nice because it elliminates the need for the dTypes list, so I can run this on any list containing any datatype. I'm not quite sure what the ast.literal_eval function exactly does though, would you care to elaborate on that?

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