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Comment by coldtea | Hacker News Reader

>The biggest gripe I have with using Python for rich command-line tool is the startup time.

What startup time?

I even have a python script running on every shell prompt drawing (that checks mercurial on top of starting the python interpreter), and the latency is negligible.

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I just tried on an app at work (Django app):

    > time python manage.py --help
    real	0m0.473s
    user	0m0.240s
    sys	0m0.148s

Half of a second is very noticeable (and annoying). I had similar perception in my previous work.

You can try to only load enough to display the help text without really loading everything, but that doesn't work that well (you have to organize things differently, and `--help` requires to load a lot of stuff. `subcommand --help` needs less but it still has to see if the subcommand exists).

As a reference:

    > time python -c 'print "hello"'
    hello

    real	0m0.041s
    user	0m0.024s
    sys	0m0.012s

    > time echo hello
    hello

    real	0m0.000s
    user	0m0.000s
    sys	0m0.000s

Actually even the first (0.041s) is not instant, while the second one does (I mean as I perceive it visually).

This is why I use Lua for command-line tools. Here's one such program that makes no attempt to organize code to make --help load faster:

  real  	0m0.006s
  user  	0m0.000s
  sys   	0m0.004s

Still, your second example is an order of magnitude faster than the example with Django. That's likely because Django loads the world on startup, parses all your model files, checks your database settings, etc.

To be fair, Python without loading site packages is pretty damn fast. Definitely faster than most other things loading up.

  $ time python -S -c 'print "Hello World!"'
  ...
  real	0m0.007s
  user	0m0.004s
  sys	0m0.003s

That's about the same speed it takes my system's cp command to show me an error page:

  $ time cp -fail
  ...
  real	0m0.005s
  user	0m0.003s
  sys	0m0.002s

That is your Django app, not Python itself, putting in that overhead.

Calling a shell built-in isn't really fair -- you should at least compare to /bin/echo (on my system the relevant times are [edit: 0.001s] for echo, 0.007+0.004=0.011 for python -S -c "print 'hello'". Without dropping site-packages (without the -S) python jumps to 0.014+0.012=0.026 -- marginally above 200 ms which I suppose is a perceptible difference between instant, and not-quite-instant).

Sadly, both pypy and python3 are slower than python2.7. Also worth nothing that the sys time fluctates for me -- in other words when it is > 0.00s it doesn't appear to have anything to do with the command run.

> Half of a second is very noticeable (and annoying). I had similar perception in my previous work.

That's because it has to load django and the whole django project. Here's click:

    > time python test.py --help
    Usage: test.py [OPTIONS]

    Options:
      --help  Show this message and exit.
    python test.py --help  0.06s user 0.02s system 95% cpu 0.078 total

and argparse:

    > time python test.py --help
    usage: test.py [-h]

    optional arguments:
      -h, --help  show this help message and exit
    python test.py --help  0.03s user 0.02s system 93% cpu 0.054 total

reference:

    > time python -c 'print "hello"'
     hello
    python -c 'print "hello"'  0.01s user 0.01s system 88% cpu 0.025 total

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