Eval | Technology Tales

TOPIC: EVAL

Controlling the version of Python used in the Positron console with virtual environments

21^st October 2025

Because I have Homebrew installed on my Linux system for getting Hugo and LanguageTool on there, I also have a later version of Python than is available from the Linux Mint repositories. Both 3.12 and 3.13 are on my machine as a consequence. Here is the line in my .bashrc file that makes that happen:

eval "$(/home/linuxbrew/.linuxbrew/bin/brew shellenv)"

The result is when I issue the command which python3, this is what I get:

/home/linuxbrew/.linuxbrew/bin/python3

However, Positron looks to /usr/bin/python3 by default. Since this can get confusing, setting a virtual environment has its uses as long as you create it with the intended Python version. This is how you can do it, even if I needed to use sudo mode for some reason:

python3 -m venv .venv

When working solely on the command line, activating it becomes a necessity, adding another manual step to a mind that had resisted all this until recently:

source .venv/bin/activate

Thankfully, just issuing the deactivate command will do the eponymous action. Even better, just opening a folder with a venv in Positron saves you from issuing the extra commands and grants you the desired Python version in the console that it opens. Having run into some clashes between package versions, I am beginning to appreciate having a dedicated environment for a set of Python scripts, especially when an IDE makes it easy to work with such an arrangement.

AND & OR, a cautionary tale

27^th March 2009

The inspiration for this post is a situation where having the string "OR" or "AND" as an input to a piece of SAS Macro code, breaking a program that I had written. Here is a simplified example of what I was doing:

%macro test;
    %let doms=GE GT NE LT LE AND OR;
    %let lv_count=1;
    %do %while (%scan(&doms,&lv_count,' ') ne );
        %put &lv_count;
        %let lv_count=%eval(&lv_count+1);
    %end;
%mend test;

%test;

The loop proceeds well until the string "AND" is met and "OR" has the same effect. The result is the following message appears in the log:

ERROR: A character operand was found in the %EVAL function or %IF condition where a numeric operand is required. The condition was: %scan(&doms,&lv_count,' ') ne ERROR: The condition in the %DO %WHILE loop, , yielded an invalid or missing value, . The macro will stop executing. ERROR: The macro TEST will stop executing.

Both AND & OR (case doesn't matter, but I am sticking with upper case for sake of clarity) seem to be reserved words in a macro DO WHILE loop, while equality mnemonics like GE cause no problem. Perhaps, the fact that and equality operator is already in the expression helps. Regardless, the fix is simple:

%macro test;
    %let doms=GE GT NE LT LE AND OR;
    %let lv_count=1;
    %do %while ("%scan(&doms,&lv_count,' ')" ne "");
        %put &lv_count;
        %let lv_count=%eval(&lv_count+1);
    %end;
%mend test;

%test;

Now none of the strings extracted from the macro variable &DOMS will appear as bare words and confuse the SAS Macro processor, but you do have to make sure that you are testing for the null string ("" or '') or you'll send your program into an infinite loop, always a potential problem with DO WHILE loops so they need to be used with care. All in all, an odd-looking message gets an easy solution without recourse to macro quoting functions like %NRSTR or %SUPERQ.