Avoiding variables as variable or list names in Python

Question

I'd like to read in a number of text files that have the following structure:

The first column specifies conditions of a behavioral experiment, the second column reaction times. What I'd like to do is to create an array/list for each condition that contains the reaction times for this condition only. I've previously used Perl for this. As there are many different conditions, I wanted to avoid writing many separate elsif statements and therefore used the condition name in the array name:

push(@{condition.${cond}}, $rt);  # $cond being the condition, $rt being the reaction time

For example, creating an array like this:

@condition3 = (560, 640,...);

As I got interested in Python, I wanted to see how I would accomplish this using Python. I found a number of answers discouraging the use of variables in variable names, but have to admit that from these answers I could not derive how I would create lists as described above without reverting to separate elif's for each condition. Is there a way to do this? Any suggestions would be greatly appreciated!

Thomas

Great! Thank you all very much for your quick and detailed help! I had not realized that it is possible to append values to existing keys in a dictionary, as all dictionary examples I had seen so far associated keys with only a single value. — Troas
– Troas, Commented Mar 30, 2012 at 22:26
Dictionaries only hold a single value for each key, but that single value can be a list. — kindall
– kindall, Commented Mar 30, 2012 at 22:48

mechanical_meat · Accepted Answer · 2012-03-30 21:37:30Z

3

A dictionary would be a good way to do this. Try something like this:

from collections import defaultdict

conditions = defaultdict(list)

for cond, rt in data:
    conditions[cond].append(rt)

edited Mar 30, 2012 at 21:37

mechanical_meat

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answered Mar 30, 2012 at 21:37

Knio

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Simeon Visser · Accepted Answer · 2012-03-30 21:39:03Z

3

The following code reads the file data.txt with the format you described and computes a dictionary with the reaction times per experiment:

experiments = {}
with open('data.txt', 'r') as f:
    data = [l.strip() for l in f.readlines()]
for line in data:
    index, value = line.split()
    try:
        experiments[int(index)].append(value)
    except KeyError:
        experiments[int(index)] = [value]
print experiments
# prints: {2: ['680'], 3: ['560', '640'], 4: ['620'], 7: ['470']}

You can now access the reaction times per experiment using experiments[2], experiments[3], et cetera.

answered Mar 30, 2012 at 21:39

Simeon Visser

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Andrew Clark · Accepted Answer · 2012-03-30 21:38:46Z

1

This is a perfect application for a dictionary, which is similar to a Perl hash:

data = {}
with open('data.txt') as f:
    for line in f:
        try:
            condition, value = map(int, line.strip().split())
            data.setdefault(condition, []).append(value)
        except Exception:
            print 'Bad format for line'

Now you can access your different conditions by indexing data:

>>> data
{2: [680], 3: [560, 640], 4: [620], 7: [470]}
>>> data[3]
[560, 640]

answered Mar 30, 2012 at 21:38

Andrew Clark

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Comments

ajmartin · Accepted Answer · 2012-03-30 21:43:43Z

1

I am not sure about your question, as to why would you think about using elif conditions.

If you store a list of integers in a dictionary, the key being values of the first column a.k.a condition value, and its corresponding value a list of reaction times.

For example: The dict would be like:

conditions['3'] -> [560, 640, ...]

answered Mar 30, 2012 at 21:43

ajmartin

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