Extracting variables from set of strings in python

Question

I have a set of strings that I extracted and stored into an array. Some of these contain variables that I want to extract, the problem is these are strings at the moment. So for example, one string would look like

s1 = 'DataFile=scan1.dat'

Others would look like

s2 = 'NumberOfChannels=32'

What I want to do is take s1 and s2 and extract from them info and store them as such:

DataFile='scan1.dat'
NumberOfChannels=32

The strings all look similar to the ones above. It's either a variable that contains a string itself, or a variable containing an integer. Here's my (failed) attempt at the integer case:

            # ll is a string element of the big array

            if ll.startswith('NumberOfChannels'): 
                print "found nChans"
                idx = ll.index('=')
                vars()[ll[:idx-1]] = int(ll[idx+1:])

Any suggestions/external modules that could help me out would be greatly appreciated

It is a common question, but this practice is frowned upon in Python because it is not only dangerous (you can easily shadow a builtin) but also an horrid namespace pollution. Store key/values in a dict, a separate namespace (or use a dummy object as a namespace). — Paulo Scardine
– Paulo Scardine, Commented Mar 1, 2014 at 20:05

Hugh Bothwell · Accepted Answer · 2014-03-01 20:08:50Z

2

def convert(val):
    """
    Attempt to coerce type of val in following order: int, float, str
    """
    for type in (int, float, str):
        try:
            return type(val)
        except ValueError:
            pass
    return val

def make_config(*args, **kwargs):
    for arg in args:
        try:
            key,val = arg.split('=')
            kwargs[key.strip()] = convert(val.strip())
        except ValueError:   # .split() didn't return two items
            pass
    return kwargs

s1 = 'DataFile=scan1.dat'
s2 = 'NumberOfChannels=32'
config = make_config(s1, s2)

which gives you

{'DataFile': 'scan1.dat', 'NumberOfChannels': 32}

edited Mar 1, 2014 at 20:08

answered Mar 1, 2014 at 20:01

Hugh Bothwell

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1 Comment

Mahi Over a year ago

+1 for this for being so easy to use, but why do you attempt to convert to str in the convert() function? The value is always str by default

Nitish · Accepted Answer · 2014-03-01 20:02:31Z

1

While this is might not be the best way of achieving whatever you are trying, here is a way of doing it:

string_of_interest = 'NumberOfChannels=32'
string_split = string_of_interest.split('=')
globals() [ string_split[0] ] = string_split[1]

EDIT: (From comments), you can try

globals() [ string_split[0] ] = eval(string_split[1])

if you trust the type conversion.

edited Mar 1, 2014 at 20:02

answered Mar 1, 2014 at 19:48

Nitish

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3 Comments

isedev Over a year ago

alternatively, eval(ll,globals()) if you trust the input.

Roland Smith Over a year ago

@isedev Have you tried using eval() in this case? It produces a SyntaxError exception.

isedev Over a year ago

yeah, it would for the string, not the integer. Good point :)

Roland Smith · Accepted Answer · 2014-03-01 20:05:13Z

With a slight modification, you can use exec for this;

In [1]: exec('NumberOfChannels = 32')

In [2]: NumberOfChannels
Out[2]: 32

In [3]: exec('DataFile="scan1.dat"')

In [4]: DataFile
Out[4]: 'scan1.dat'

Note that you should never feed untrusted code to exec!

And loading random variables into your program is generally a bad idea. What if the variable names are the same as some used in your program?

If you want to store configuration data, have a look at json instead.

In [6]: import json

In [7]: json.dumps({'NumberOfChannels': 32, 'DataFile': 'scan1.dat'})
Out[7]: '{"DataFile": "scan1.dat", "NumberOfChannels": 32}'

In [8]: json.loads('{"DataFile": "scan1.dat", "NumberOfChannels": 32}')
Out[8]: {u'DataFile': u'scan1.dat', u'NumberOfChannels': 32}

In [9]: config = json.loads('{"DataFile": "scan1.dat", "NumberOfChannels": 32}')

In [10]: config['DataFile']
Out[10]: u'scan1.dat'

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