Python Entry Points Explained

rajathagasthya|8 years ago

I highly recommend reading Stevedore package's excellent documentation [1] to see practical examples of how entry points can be used as a plugin manager in applications.

[1] - https://docs.openstack.org/stevedore/latest/index.html

jwilk|8 years ago

Entry points are excellent at making your scripts start slooooooooow.

Compare

  $ time python -m pyflakes /dev/null
  
  real	0m0.225s
  user	0m0.103s
  sys	0m0.008s

with

  $ time pyflakes /dev/null
  
  real	0m2.468s
  user	0m1.738s
  sys	0m0.055s

ericfrederich|8 years ago

I did not get as dramatic of results as you.

  # python3 -m venv example
  # source example/bin/activate
  (example) # pip install pyflakes
  Collecting pyflakes
    Downloading pyflakes-1.5.0-py2.py3-none-any.whl (225kB)
      100% || 225kB 892kB/s 
  Installing collected packages: pyflakes
  Successfully installed pyflakes-1.5.0
  (example) # time for i in {00..99}; do python -m pyflakes /dev/null; done
  real    0m7.167s
  user    0m6.407s
  sys 0m0.750s
  (example) # time for i in {00..99}; do pyflakes /dev/null; done
  
  real    0m7.661s
  user    0m6.894s
  sys 0m0.753s

gizmo385|8 years ago

I've noticed that using argparse makes my scripts start very slow, especially if you expose a large number of option. I have a script that I wrote for work that has quite a variable number of available options and it can sometimes take several seconds for the script to startup and actually begin doing anything useful.

pmoriarty|8 years ago

Way too much ink wasted on cuteness and too little on explanation.

I got stumped at:

"In particular, the magic happens in get_sneks. The call to pkg_resources.iter_entry_points('snek_types') iterates over all the entry points that were registered under the name "snek_types". So, external packages can define an entry point called "snek_types" in their setup.py, and snek will dynamically load it at runtime."

Wait. What entry points were registered under the name "snek_types"? Where were they regeistered as such? I think I must be missing something, or maybe that registration was hidden somewhere among all the fancy snakes. Can someone help explain this in a clearer way?

antoncohen|8 years ago

"Snek Pro Solutions" distributes a package (pip installed) called cute-snek. In the cute-snek setup.py it has:

    entry_points={
        'snek_types': [
            'cute = cute_snek:cute_snek',
        ],
    }

The main snek codebase has:

    for entry_point in pkg_resources.iter_entry_points('snek_types'):
        sneks[entry_point.name] = entry_point.load()

pkg_resources.iter_entry_points() iterates through all the snek_types an end user has installed on their system. When the end user does `pip install snek` they will only have the built-in sneks, when they also do `pip install cute-snek` the will have the "cute" snek installed, allowing them to run `snek --type cute` to see it.

greenshackle2|8 years ago

You have to keep reading, further down they show the updated setup.py with the snek_types entry points.

I agree it's not super well organized, they should have printed the new setup.py before that bit.

lftl|8 years ago

That seemed like magic to me at first too. I think though that is happens during the installation of the package, which is why he had to reinstall after refactoring.

I initially just skimmed over his log dump from the install, but I think the details are actually important there:

> writing entry points to cute_snek.egg-info\entry_points.txt

Karunamon|8 years ago

The registration is handled by the contents of the tuple in setup.py. Specifically:

    from setuptools import setup

    setup(
        name='snek',
            entry_points={
            'console_scripts': [
                'snek = snek:main',
            ],
        }
    )

It's using the setuptools module, and passing a tuple to it - which is used to create executables for the individual scripts once installed.

https://packaging.python.org/tutorials/distributing-packages...

Specifically, "entry_points" in the setup tuple is what sets the name of the created files and links them to your code. In the example, the executable 'snek' points to main() in snek.py.

Once your package is installed, pip or easy_install handles the magic of putting those files in place. There are boatloads of other options available (much like any packaging system), but this is a minimum viable example.

sjburt|8 years ago

What is the actual value of entry points over just marking a script +x and putting somewhere on the path? All I see is a bunch of added complexity.

rcthompson|8 years ago

Answering this question was pretty much the entire point of the article. I felt that each section gave a concrete example of a situation where the solution in the previous section fell short and then introducing some new complexity to handle it.

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