Can a variable expansion in python work like shell expansion of variable

1
votes

I have a legacy python code that reads from a shell script for variable values like in a properties file. For example, say, I have a shell program x.sh for variable declaration as:

Y_HOME=/utils
Y_SPCL=$Y_HOME/spcl
UTIL1=$Y_SPCL/u1
Y_LIB=$Y_HOME/lib

Now, from within python program abc.py, I read x.sh file line by line and use line.split("=")[1] to get the value of the variable, say, UTIL1 as $Y_SPCL/u1 in non-expanded form and not in expanded form as /utils/spcl/u1. Can I have sone mechanism in python to have vafiable expandion like in a shell program execution. I think, since I am using x.sh not as a shell program, rather as a configuration file like properties, there should be all variables in expanded form to let the python program run properly, such as:

Y_HOME=/utils
Y_SPCL=/utils/spcl
UTIL1=/utils/spcl/u1
Y_LIB=/utils/lib

This will have no change on the legacy python part of code and changing the configuration file as an external properties data. Please pass your opinions.

2
pythonshellsplitvariable-expansion
Why is it easier to implement this in code, rather than just changing the config file to be like your second example? - jordanm
You can have a shell interpret the file, and then query it for resulting variables - that other guy
Is there a good reason why you are parsing the config file yourself in hand-crafted code instead of using the module configparser? And whether you do that or not, you can use os.path.expandvars() to do what you want. - BoarGules

2 Answers

3
votes

There is a package dotenv that can do that.

pip install --user python-dotenv

Then, in Python:

import dotenv, os
dotenv.load_env("x.sh")
print(os.environ["Y_LIB"])

Important: Make sure your variable substitutions read like ${VAR}. So your x.sh would look like this:

Y_HOME=/utils
Y_SPCL=${Y_HOME}/spcl
UTIL1=${Y_SPCL}/u1
Y_LIB=${Y_HOME}/lib
1
votes

Assuming variables must be declared before use and they form a correct path when expanded, you could do something like this.

There is the file fileName that contains the variables:

Y_HOME=/utils
Y_SPCL=$Y_HOME/spcl
UTIL1=$Y_SPCL/u1
Y_LIB=$Y_HOME/lib

So, For each "variable", you search for it in the next variables "values" and replace it for the proper "value". You could have a .py like this:

variables = []
with open("fileName", 'r') as f:
    while True:
        line = f.readline()[:-1] # Get the line minus '\n'
        if not line:
            break
        variables.append(line.split('=')) 

    for i in range(len(variables)):
        current =  "$" + variables[i][0]
        for j in range(len(variables)):
            replaced = None
            while replaced != variables[j][1]:
                # We replace until no further replaces happen
                replaced = variables[j][1].replace(current, variables[i][1])
                variables[j][1] = replaced

for var in variables:
    print(var[0] + "=" + var[1])

Output:

Y_HOME=/utils
Y_SPCL=/utils/spcl
UTIL1=/utils/spcl/u1
Y_LIB=/utils/li