Assign variable names and data in a loop

Question

I'm trying to load data into datasets in Python. My data is arranged by years. I want to assign variable names in a loop. Here's what it should look like in pseudocode:

import pandas as pd
for i in range(2010,2017):
    Data+i = pd.read_csv("Data_from_" +str(i) + ".csv")
    # Stores data from file "Data_from_YYYY.csv" as dataset DataYYYY.

The resulting datasets would be Data2010 - Data2017.

You can't. Well, at least, you definitely shouldn't (using exec is dangerous). What's wrong with a dictionary data = {}; for i in range(2010, 2017): data[i] = pd.read_csv("Data_from_" +str(i) + ".csv")? — FHTMitchell
– FHTMitchell, Commented Mar 7, 2018 at 19:18
There are canonical ways which aren't risky the way that exec is, see my answer. — jooks
– jooks, Commented Mar 7, 2018 at 19:26

Aaron Christiansen · Accepted Answer · 2018-03-07 19:27:40Z

3

While this is possible, it is not a good idea. Code with dynamically-generated variable names is difficult to read and maintain. That said, you could do it using the exec function, which executes code from a string. This would allow you to dynamically construct your variables names using string concatenation.

However, you really should use a dictionary instead. This gives you an object with dynamically-named keys, which is much more suited to your purposes. For example:

import pandas as pd
Data = {}
for i in range(2010,2017):
    Data[i] = pd.read_csv("Data_from_" +str(i) + ".csv")
    # Stores data from file "Data_from_YYYY.csv" as dataset DataYYYY.

# Access data like this:
Data[2011]

You should also use snake_case for variable names in Python, so Data should be data.

If you really wanted to dynamically generate variables, you could do it like this. (But you aren't going to, right?)

import pandas as pd
for i in range(2010,2017):
    exec("Data{} = pd.read_csv(\"Data_from_\" +str(i) + \".csv\")".format(i))
    # Stores data from file "Data_from_YYYY.csv" as dataset DataYYYY.

You can do this without exec too; have a look at Jooks' answer.

edited Mar 7, 2018 at 19:27

answered Mar 7, 2018 at 19:19

Aaron Christiansen

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5 Comments

Drise Over a year ago

Code with dynamically-generated variable names is difficult to read and maintain. This can't be said loudly enough.

jooks Over a year ago

To wit: python uses snake_case, not camelCase.

Aaron Christiansen Over a year ago

@jooks Whoops, I'll change my answer. Been doing much more C# lately!

Drise Over a year ago

@AaronChristiansen Relevant PEP 8 section regarding use of snake_case

Quentin Over a year ago

Haven't seen exec() before. Cool. Also, code with dynamically-generated variables sounds like a footgun.

jooks · Accepted Answer · 2018-03-07 19:24:34Z

1

You can use globals, locals or a __init__ which uses setattr on self to assign the variables to an instance.

In [1]: globals()['data' + 'a'] = 'n'

In [2]: print dataa
n

In [3]: locals()['data' + 'b'] = 'n'

In [4]: print datab
n

In [5]: class Data(object):
   ...:     def __init__(self, **kwargs):
   ...:         for k, v in kwargs.items():
   ...:             setattr(self, k, v)
   ...:

In [6]: my_data = Data(a=1, b=2)

In [7]: my_data.a
Out[7]: 1

I would probably go the third route. You may be approaching your solution in an unconventional way, as this pattern is not very familiar, even if it is possible.

answered Mar 7, 2018 at 19:24

jooks

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