It would be simpler to expand variables from the whole file before even splitting it into lines than doing that for each line. It would make it impossible to expand variables only from some parts of the lines, but I actually don't think that's a good idea in general. I'll comment that separately later.

Should we also support #expandvars:? What about # expandvars: where the space is actually (and most likely accidentally) a non-break space? An easy way to support them would be using regular expressions. They are often complicated, but in this case I consider the resulting code simpler:

match = re.fullmatch('#\s*expandvars:\s*(.*?)\s*', lines[0], flags=re.IGNORECASE)
if not match:
    return False
value = match.group(1).lower()
return value not in ("false", "no", "off", "")

i think that would be : .upper() if we are goind to use FALSE_STRINGS instead of ("false", "no", "off", "")
otherwise , resolved

This implementation expands variable only from option values, not from option names. I don't think that's a good idea for the following reasons:

1. On the commend line you can use --$name value just fine so the implementation is inconsistent with that. I see no problems with supporting variables in option names either.
2. The implementation doesn't work correctly when using short options without a separator like -N$value. Trying to fix this would be pretty complicated, because short options can be combined like -XN$value. Related to this, at least the -N$value usage should get a test!
3. If variables are parsed everywhere, it can be done before splitting content to lines which simplifies the overall implementation.
4. If variables are parsed as the first step, it allows conditional usage like

   $COND --name Example
   

   where $COND could be set to # to exclude the line. Not sure is this that useful, but could come handy in some cases.

1. Using Template.safe_substitute instead of os.path.expandvars is probably a good idea, but the latter would support %name% substitution on Windows. If that's important, we ought to be able to to extend Template to support it.
2. One benefit of using Template is that we could extend it to support ${name=default} style somewhat easily. It could even be part of this enhancement, but it can also be added later.
3. No need to explain how safe_substiture works here.

thanks for the review

i see the trade-oof between using Template.safe_substitute and os.path.expandvars

1- i thought a bit about extending Template to support both ${var} and %var% formats, the solution i saw is :
creating like " EnvVarTemplate subclass " that extend functionality through inheritance to support %var% formats. I think that this would require regex pattern. however, I'm still wondering if adding windows style variable support critical for the current enhancement? I think that i can push this enhancement later since it will requires more tests

2- i' am considering adding this support ( ${name=default} ) but i don't see another solution other then this :

    def _expand_vars(self, value):
        result = Template(value).safe_substitute(os.environ)
        regexpatter, = r'\${([^}]+?)=([^}]*)}'
        def replace_with_default_value(match):
            var, default = match.groups()
            return os.environ.get(var, default)
        return re.sub(regexpatter, replace_with_default_value, result)

3- intended to make a comment for design reminder