pyenv init slows down the shell startup a bunch (by about ~100 ms!)

[VorpalBlade]

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Description

• Platform information (e.g. Ubuntu Linux 16.04): Arch Linux
• OS architecture (e.g. amd64): amd64
• pyenv version: 2.3.36
• Python version: 3.11.7 (on host)
• C Compiler information (e.g. gcc 7.3): 13.2.1 20230801
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pyenv init is really slow. I have been profiling zsh startup time, and commenting out eval "$(pyenv init -)" reduces startup time from 111.7 ms to just 11.2 ms. I have been using hyperfine for statistical rigour in the measurement (full output below).

• I could possibly just inline the output of pyenv init - directly into my .zshrc. But that only reduces the startup time by about ~25 ms (down to 86 ms). And it also risks breaking if pyenv updates.
• The output of pyenv init --path is just as slow to process as the output of pyenv init - by the way. So that makes absolutely no difference.
• pyenv init in itself takes 30 ms, but since ditching it and inlining only saved 25 ms I suspect that the shell is able to start processing the output of eval as pyenv is writing it (I guess pyenv writes it kind of slowly line by line rather than a single write()?).

I believe a large part of the problem is that the pyenv output starts a bash sub-process to process some code. This is known to be slow, and is why /bin/sh on Debian for example is dash not bash. Since I'm on Arch it wouldn't help to switch to /bin/sh (which is bash here). Really you should do all your logic directly in the shell you are running in, not another one.

Ideally I would suggest that instead of a command like this, pyenv should install a file you can source (and that doesn't call so many commands, and definitely no subshells). Possibly you need one such installed file per shell you support. I'm not sure about this.

Here are how I measured the shell startup (time to first accepted command for an interactive shell):

$ # With pyenv init in .zshrc
$ hyperfine -i -N --warmup 1 "zsh -i -c exit"          
Benchmark 1: zsh -i -c exit
  Time (mean ± σ):     111.7 ms ±   1.0 ms    [User: 85.4 ms, System: 42.1 ms]
  Range (min … max):   110.1 ms … 115.3 ms    27 runs

$ # With pyenv commented out
$ hyperfine -i -N --warmup 1 "zsh -i -c exit"
Benchmark 1: zsh -i -c exit
  Time (mean ± σ):      11.2 ms ±   0.2 ms    [User: 6.0 ms, System: 5.2 ms]
  Range (min … max):    10.8 ms …  12.2 ms    251 runs

[VorpalBlade]

Just checked on my laptop. There the difference is larger (it is a slower and older computer): 160.4 ms down to 16.5 ms.

[aalbaali]

I have the same issue as well on zsh. Running pyenv init - or $(pyenv init --path) adds around 300 ms (I go from 80 ms to 380).

[VorpalBlade]

I figured out a workaround:

pyenv() {
    eval "$(command pyenv init -)"

    pyenv "$@"
}

This will initialise itself on first use and overwrite with the slow implementation. As I don't use pyenv very often (I mostly do Rust these days) this works for me. For even less overhead, put that in a zsh autoloadable file so it isn't even parsed until it is needed.