Preprocessor commands escape if condition

[Daniel Kraehmer]

Dear Statalist community,

In the code below, both #review and #delimit are executed although the if condition is not fulfilled.

clear all
sysuse auto.dta, clear

local delimchange 0

if `delimchange' == 1 {
    #review
    #delimit ;
}

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tab weight

Is this intended behavior? I know that both #review and #delimit are preprocessor commands, but I couldn't find much information in the documentation.

The issue initially came up when I integrated Python code into my do-file, using # to start a comment.

clear all

qui python query
if `r(initialized)' != 1 {
    python search
    set python_exec  "C:my/dummy/path/python.exe"    // set path
}
 
local py 0

if `py' == 1 {
python:
# Here comes a simple print command
print("Hello world")
end
}

I am using Stata 18 (update level: 26 Feb 2025) on Windows 11.

[Leonardo Guizzetti]

I think the first part of your question regarding pre-processor commands is a of a diversion from what I suspect you are really interested in, which is how to conditionally execute your python block.

As it happens, when you have a code block, in this case your -if- command, you cannot have nested language blocks (e.g., -python {- or -python:-) like you could in open code outside of such a block. To execute Python-specific commands like that, you must use the -python:- prefix for every single Python command. In your case, this would become:

if `py' == 1 {
    python: # Here comes a simple print command
    python: print("Hello world")
}

The same is true if, instead of Python, you wanted to execute Mata statements. Some helpful related discussions from the past can be found here.

[Daniel Kraehmer]

I'm not sure about your comment on nested language blocks inside code blocks. The following works just fine for me:

local py 1

if `py' == 1 {
python:
print("1st line")
print("2nd line")
print("3rd line")
end
}

My question was more generally about preprocessing commands and why they are considered by Stata even when they are tucked away in an unfulfilled if condition. The Python case was just the first time I had encountered this behavior.

[Leonardo Guizzetti]

I'm speculating here, but when executing code from the do-file editor, or an ado, I think each line of the file is first preprocessed before being processed as actual code. The preprocessor must see lines beginning with "#" as a preprocessor directive and tries to execute those immediately, rather than treating them like regular Stata commands that follow the same logical flow implied by the program.