Using a text file to receive a string for a variable in Python, without defining it

Question

I have a text file in which there are several variables. Most of them are used in a Bash script of mine, but I'd like to use the same text file for my Python script. For the lines that are not properly formatted for Python, I want my script to just ignore. For those that are properly formatted, I want the script to check and if it's the variable I'm looking for - use it.

import sys import re

for ln in open("thefile.txt"):
        m = re.match(r"(?P<varname>[^=]*)\s*=\s*(?P<value>.+)", ln)
        if m:
                varname = m.group("varname")
                value_string = m.group("value")
                value = eval(value_string)
                print value
                # so if the variables name is THISVARIABLE, get that value:
                if varname == "THISVARIABLE":
                        mypythonvariable == value

I'm getting the following error:

NameError: name 'Somevariableinmytextfile' is not defined

The Somevariableinmytextfile is the first variable in that file.

My question:

Do I have to define every variable in the txt file, in order to get rid of this error? If not, what shall I do? I'm very new at Python. This is my first program.

What's the actual error you're getting? I doubt it literally says 'Somevariableinmytextfile'. — user2357112
– user2357112, Commented Jul 21, 2013 at 21:56
The "somevarianble..." is the first variable in my list. At the moment "infothing". — Paolo
– Paolo, Commented Jul 22, 2013 at 12:23
A few examples from thefile.txt (one on each row, although I can't make a line break here): infothing=/home/box/ infonumber=125 — Paolo
– Paolo, Commented Jul 22, 2013 at 12:24

michaelb958--GoFundMonica · Accepted Answer · 2013-07-22 03:04:10Z

1

The error is eval complaining that the contents of value_string have no meaning as a whatever-it-is.

The real error is using eval at all. (A good post on the pitfalls can be found here.) You don't even need to eval here - leaving value_string as the string the regex gave you will be just fine.

The problem with the present approach

Sample thefile.txt:

foo=bar
baz=42
quux=import os; os.shutdown()

When parsing foo, Python complains that bar isn't defined. (Simple.)
When parsing bar, Python gives you an int instead of a str. (No real problem...)
When parsing quux, Python shuts down your computer. (Uh oh!)

Why you don't need `eval`

You want a string value, correct? The regex already gives you a string!

varname = m.group("varname")
value = m.group("value")
print value
if varname == "THISVARIABLE":
    mypythonvariable = value # You meant = instead of ==?

answered Jul 22, 2013 at 3:04

michaelb958--GoFundMonica

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6 Comments

Paolo Over a year ago

I did mean = (assign) rather than == (compare). Yes. And thank you very much for a great answer!

michaelb958--GoFundMonica Over a year ago

@PaoloVacirca The code, with my fixes, works fine for me on Python 2.7.3 (WinRT).

Paolo Over a year ago

It turns out it's not assigning any value to THISVARIABLE. It's empty. There's something else wrong here, on my end of it. I'm trying to figure out what. :)

michaelb958--GoFundMonica Over a year ago

@PaoloVacirca Perhaps check if mypythonvariable is empty? (That's what the sample code is assigning to...)

Paolo Over a year ago

It is empty. I don't understand why though. I must have missed something in the loop.

|

nio · Accepted Answer · 2013-07-21 22:08:40Z

0

You get the error because a line in your thefile.txt file:

TEST = ABC

will be evaluated in python as TEST will be assigned to a value of variable ABC but you have no ABC defined.

You could make a dictionary to store your value pairs... this will work with string values:

variables = {}
accepted = ['THISVARIABLE', 'ANOTHERONE']
...
if varname in accepted:
    variables[varname]=value

edited Jul 21, 2013 at 22:08

answered Jul 21, 2013 at 22:00

nio

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Comments

P0W · Accepted Answer · 2013-07-21 22:17:41Z

0

eval throws the error, the value_string's value (which should be a variable) needs to be defined before use.

edited Jul 21, 2013 at 22:17

answered Jul 21, 2013 at 22:08

P0W

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