Using a string in command line input (python) to refer to a dictionary

Question

I have a few pre-defined dictionaries in my python code, e.g.:

Parameter_A = {'Alpha':0.31, 'Beta':0.69}
Parameter_B = {'Alpha':0.55, 'Beta':0.75}

During execution of the python script (foo.py) I refer to the chosen parameter dictionary that I want to use (i.e. Parameter_A or Parameter_B). Like:

python foo.py A

And in the python script I have:

var = sys.argv[1]
Param = 'Parameter_' + var

The line (in script)

print 'Param = ', Param

gives output:

Param = Parameter_A

But, when I try to access the elements in the dictionary using this method,

Param['Alpha']

I get the error:

NameError: name 'Param' is not defined

I could see that in this method 'Param' (which has the string Parameter_A as its value) is of type 'str'.

But, I will really appreciate if someone can tell me how I can take 'A' or 'B' as a command line argument to indicate which one of the two dictionaries (Parameter_A or Parameter_B) my code should use.

John La Rooy · Accepted Answer · 2017-02-24 06:34:40Z

1

Don't use eval - it's potentially dangerous (depending who supplies the arguments to your program)

you could look them up in globals()

But why not use a dict for Parameter?

Parameter = {'A': {'Alpha':0.31, 'Beta':0.69},
             'B': {'Alpha':0.55, 'Beta':0.75}}
Param = Parameter[sys.argv[1]]

answered Feb 24, 2017 at 6:34

John La Rooy

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frederick99 · Accepted Answer · 2017-02-24 05:14:55Z

1

You can use eval() bif.

import sys

Parameter_A = {'Alpha':0.31, 'Beta':0.69}
Parameter_B = {'Alpha':0.55, 'Beta':0.75}

var = sys.argv[1]
Param = 'Parameter_' + var

d = eval(Param)

print(d['Alpha'])

Terminal
$ python foo.py A

Output
0.31

answered Feb 24, 2017 at 5:14

frederick99

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Comments

Renjith Thankachan · Accepted Answer · 2017-02-24 05:24:18Z

Note that Param = 'Parameter_' + var will only create a string Parameter_A not value of Parameter_A is assigned or refered anyways, on reading your code, to create something like that, you can use globals or eval.

Using globals

  var = sys.argv[1]
  Param = globals()['Parameter_' + var]  
  # here you are refering `Prameter_A` as `Param`
  print 'Param = ', Param  
  print 'Param[Alpha] = ', Param['Alpha']

Using eval

  var = sys.argv[1]
  # eval excecutes the string as python code
  # so if var is `A;import os;os.remove('somefile')`
  # will remove the file
  Param = eval('Parameter_' + var)  
  # here you are refering `Prameter_A` as `Param`
  print 'Param = ', Param  
  print 'Param[Alpha] = ', Param['Alpha']

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