Add ability for ``__post_init__`` method to be defined. #111

tbeadle · Nov 11, 2016

If this method is defined for a class, it will get executed at the end
of __init__. Previously, there was no way to extend what was
done during __init__ when using @attr.s.

hynek · Nov 11, 2016

hynek reviewed Nov 11, 2016

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hynek · Nov 11, 2016

src/attr/_make.py

@@ -680,6 +680,10 @@ def fmt_setter_with_converter(attr_name, value_var):
                                                    a.name))
        names_for_globals[val_name] = a.validator
        names_for_globals[attr_name] = a
+    lines.extend([
+        "func = getattr(self, '__post_init__', None)",


This is approach means that attrs gets universally slower for everyone. attrs is explicitly static in its approach. We need to check for the existence of the method when assembling the class and call it unconditionally if it’s present.

codecov-io · Nov 11, 2016

Current coverage is 100% (diff: 100%)

Merging #111 into master will not change coverage

@@           master   #111   diff @@
====================================
  Files           8      8          
  Lines         466    468     +2   
  Methods         0      0          
  Messages        0      0          
  Branches      105    106     +1   
====================================
+ Hits          466    468     +2   
  Misses          0      0          
  Partials        0      0

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hynek · Nov 11, 2016

hynek reviewed Nov 11, 2016

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hynek · Nov 11, 2016

src/attr/_make.py

+    script, globs = _attrs_to_script(
+        attrs,
+        frozen,
+        post_init=hasattr(cls, '__post_init__')


No hasattr ever please. This lends itself to getattr(cls, "__post_init__", False).

hynek · Nov 13, 2016

hynek requested changes Nov 13, 2016

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I have added a bunch of comments many of which arguably should have been covered by the contribution guide so I’ve added it to my own todo list to fix that.

If you have problems with Hypothesis, offering candy to @Tinche always helped for me. ;)

Thank you for your contribution! And please remember to ping the ticket once you’re done.

hynek · Nov 13, 2016

src/attr/_make.py

@@ -540,7 +544,7 @@ def validate(inst):
            a.validator(inst, a, getattr(inst, a.name))


-def _attrs_to_script(attrs, frozen):
+def _attrs_to_script(attrs, frozen, post_init=False):


Any reason why this has a default value? I find them indicative of problems if used outside public APIs.

hynek · Nov 13, 2016

src/attr/_make.py

@@ -680,6 +684,8 @@ def fmt_setter_with_converter(attr_name, value_var):
                                                    a.name))
        names_for_globals[val_name] = a.validator
        names_for_globals[attr_name] = a
+    if post_init:
+        lines.append("self.__post_init__()")


My guts tell me the post init should be called before validating the object. But I’d love input from people who actually care about this feature. Summoning all the bikeshed artists from #68: @glyph @Insoleet @wearpants @Tinche @wbolster @mathieulongtin @ambv

Speak now or forever hold your peace.

I'd say post validation. I'd expect __post_init__ to be the very last thing init calls. Beside, we can call attr.validate.

I'm voting post validation/conversion. The name __post_init__ itself suggests it's called after the generated __init__, so I'd expect fields to be validated, converted and set. Having it called post-conversion lowers the amount of possible states we can expect the object to be in.

So be it.

We’ll have to add the pass-self-to-factories thing then because otherwise there’s no way to validate dependent attributes.

The other way to validate dependent attributes would be for __post_init__ to raise an exception, no?

Sure but I like the idea of separating the factory process from the validation process.

hynek · Nov 13, 2016

tests/test_make.py

@@ -302,6 +302,22 @@ class D(object):
        assert C.D.__name__ == "D"
        assert C.D.__qualname__ == C.__qualname__ + ".D"

+    def test_post_init(self):


While this test is fine as it is, we make sure that new features work well with the rest of attrs by making them part of our Hypothesis testing strategies: https://github.com/hynek/attrs/blob/master/tests/utils.py#L148-L171

And while squinting at this code I also realized that we should add post_init support to make_class so we have API parity! It should take a callable for argument and attach it as a method to the created class before applying attrs magic to it in https://github.com/hynek/attrs/blob/master/src/attr/_make.py#L816

make_class already supports it. A __post_init__ key can be defined with the dictionary passed in to make_class:

>>> import attr >>> def foo(self): ... print('in foo') ... >>> C1 = attr.make_class('C1', {'x': attr.ib(default=42), '__post_init__': foo}) >>> c = C1() in foo

Fair enough, please just add it to our testing strategy then.

oh wait you did never mind :) I’ll have another deeper look on the weekend

hynek · Nov 13, 2016

docs/examples.rst

@@ -551,6 +551,25 @@ You can still have power over the attributes if you pass a dictionary of name: `
   >>> i.y
   []

+Sometimes, you want to have your class's ``__init__`` method do more than just


Semantic new lines pls.

hynek · Nov 13, 2016

CHANGELOG.rst

@@ -13,6 +13,9 @@ Changes:

 - Don't overwrite ``__name__`` with ``__qualname__`` for ``attr.s(slots=True)`` classes.
  `#99 <https://github.com/hynek/attrs/issues/99>`_
+- Allow for a ``__post_init__`` method that, if defined, will get executed at


We use semantic new lines in prose. Simply make each sentence a new line please.

Also please add a link to this PR like in the entry before.

hynek · Nov 13, 2016

tests/test_make.py

+                self2.z = self2.x + self2.y
+
+        c = C(x=10, y=20)
+        assert hasattr(c, 'z')


hynek · Nov 13, 2016

tests/test_make.py

+
+        c = C(x=10, y=20)
+        assert hasattr(c, 'z')
+        assert c.z == 30


expected == actual please

wearpants · Nov 15, 2016

Late to the party, but yes absolutely post conversion

hynek · Nov 20, 2016

I’ve fixed it up so we don’t have ping-pong over minutiae anymore, thank you very much for your contribution!

I’ve decided to rename __post_init__ to __attrs_post_init__ for consistency with __attrs_attrs__ and to be explicit about it’s heritage.

If someone has good arguments against it, feel free to open a new issue before 16.3.0 is out.

tbeadle force-pushed the tbeadle:post_init branch from 6106e0d to 1ac1cdd Nov 11, 2016

tbeadle force-pushed the tbeadle:post_init branch from 1ac1cdd to 0f6636b Nov 11, 2016

Add ability for ``__post_init__`` method to be defined.
If this method is defined for a class, it will get executed at the end of ``__init__``. Previously, there was no way to extend what was done during ``__init__`` when using ``@attr.s``.

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tbeadle force-pushed the tbeadle:post_init branch from 0f6636b to 7724d31 Nov 14, 2016

hynek added this to the 16.3.0 milestone Nov 19, 2016

hynek closed this in 1fbaa20 Nov 20, 2016

hynek referenced this pull request Nov 20, 2016
Closed
init hooks #68

wsanchez referenced this pull request Feb 17, 2017
Closed
frozen class is made immutable before __attrs_post_init__ gets a turn #147

python-attrs/attrs

Add ability for ``__post_init__`` method to be defined. #111

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frozen class is made immutable before __attrs_post_init__ gets a turn #147

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Add ability for ``__post_init__`` method to be defined. #111

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Current coverage is 100% (diff: 100%)

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