The Universe of Discourse

Sun, 06 Feb 2022

A one-character omission caused my Python program to hang (not)

I just ran into a weird and annoying program behavior. I was writing a Python program, and when I ran it, it seemed to hang. Worried that it was stuck in some sort of resource-intensive loop I interrupted it, and then I got what looked like an error message from the interpreter. I tried this several more times, with the same result; I tried putting exit(0) near the top of the program to figure out where the slowdown was, but the behavior didn't change.

The real problem was that the first line which said:

    #/usr/bin/env python3

when it should have been:

    #!/usr/bin/env python3

Without that magic #! at the beginning, the file is processed not by Python but by the shell, and the first thing the shell saw was

    import re

which tells it to run the import command.

I didn't even know there was an import command. It runs an X client that waits for the user to click on a window, and then writes a dump of the window contents to a file. That's why my program seemed to hang; it was waiting for the click.

I might have picked up on this sooner if I had actually looked at the error messages:

    ./ line 9: dictionary: command not found
    ./ line 10: syntax error near unexpected token `('
    ./ line 10: `words = read_dictionary(dictionary)'

In particular, dictionary: command not found is the shell giving itself away. But I was so worried about the supposedly resource-bound program crashing my session that I didn't look at the actual output, and assumed it was Python-related syntax errors.

I don't remember making this mistake before but it seems like it would be an easy mistake to make. It might serve as a good example when explaining to nontechnical people how finicky and exacting programming can be. I think it wouldn't be hard to understand what happened.

This computer stuff is amazingly complicated. I don't know how anyone gets anything done.

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