Pass args from `__init__` to `__attrs_pre_init__` if requested (#1187)

* Pass args from `__init__` to `__attrs_pre_init__` if requested

Detect if `__attrs_pre_init__` has arguments besides `self`
using `inspect.signature`. If so, pass `__attrs_pre_init__`
the same arguments that `__init__` (or `__attrs_init__`)
expects.

* Add changelog entry for `__attrs_pre_init__` args changes

* Don't use monkeypatch in new code

* Clarify docs

* Missed one monkeypatching

---------

Co-authored-by: Hynek Schlawack <hs@ox.cx>

changelog.d/1187.change.md

@@ -0,0 +1,2 @@
+If *attrs* detects that `__attrs_pre_init__` accepts more than just `self`, it will call it with the same arguments as `__init__` was called.
+This allows you to, for example, pass arguments to `super().__init__()`.

docs/init.md

@@ -357,7 +357,8 @@ However, sometimes you need to do that one quick thing before or after your clas
 For that purpose, *attrs* offers the following options:
 
 - `__attrs_pre_init__` is automatically detected and run *before* *attrs* starts initializing.
-  This is useful if you need to inject a call to `super().__init__()`.
+  If `__attrs_pre_init__` takes more than the `self` argument, the *attrs*-generated `__init__` will call it with the same arguments it received itself.
+  This is useful if you need to inject a call to `super().__init__()` -- with or without arguments.
 
 - `__attrs_post_init__` is automatically detected and run *after* *attrs* is done initializing your instance.
   This is useful if you want to derive some attribute from others or perform some kind of validation over the whole instance.

src/attr/_make.py

@@ -3,6 +3,7 @@
 import contextlib
 import copy
 import enum
+import inspect
 import linecache
 import sys
 import types
@@ -624,6 +625,7 @@ class _ClassBuilder:
         "_delete_attribs",
         "_frozen",
         "_has_pre_init",
+        "_pre_init_has_args",
         "_has_post_init",
         "_is_exc",
         "_on_setattr",
@@ -670,6 +672,13 @@ class _ClassBuilder:
         self._weakref_slot = weakref_slot
         self._cache_hash = cache_hash
         self._has_pre_init = bool(getattr(cls, "__attrs_pre_init__", False))
+        self._pre_init_has_args = False
+        if self._has_pre_init:
+            # Check if the pre init method has more arguments than just `self`
+            # We want to pass arguments if pre init expects arguments
+            pre_init_func = cls.__attrs_pre_init__
+            pre_init_signature = inspect.signature(pre_init_func)
+            self._pre_init_has_args = len(pre_init_signature.parameters) > 1
         self._has_post_init = bool(getattr(cls, "__attrs_post_init__", False))
         self._delete_attribs = not bool(these)
         self._is_exc = is_exc
@@ -974,6 +983,7 @@ class _ClassBuilder:
                 self._cls,
                 self._attrs,
                 self._has_pre_init,
+                self._pre_init_has_args,
                 self._has_post_init,
                 self._frozen,
                 self._slots,
@@ -1000,6 +1010,7 @@ class _ClassBuilder:
                 self._cls,
                 self._attrs,
                 self._has_pre_init,
+                self._pre_init_has_args,
                 self._has_post_init,
                 self._frozen,
                 self._slots,
@@ -1984,6 +1995,7 @@ def _make_init(
     cls,
     attrs,
     pre_init,
+    pre_init_has_args,
     post_init,
     frozen,
     slots,
@@ -2027,6 +2039,7 @@ def _make_init(
         frozen,
         slots,
         pre_init,
+        pre_init_has_args,
         post_init,
         cache_hash,
         base_attr_map,
@@ -2107,6 +2120,7 @@ def _attrs_to_init_script(
     frozen,
     slots,
     pre_init,
+    pre_init_has_args,
     post_init,
     cache_hash,
     base_attr_map,
@@ -2361,11 +2375,23 @@ def _attrs_to_init_script(
         lines.append(f"BaseException.__init__(self, {vals})")
 
     args = ", ".join(args)
+    pre_init_args = args
     if kw_only_args:
         args += "%s*, %s" % (
             ", " if args else "",  # leading comma
             ", ".join(kw_only_args),  # kw_only args
         )
+        pre_init_kw_only_args = ", ".join(
+            ["%s=%s" % (kw_arg, kw_arg) for kw_arg in kw_only_args]
+        )
+        pre_init_args += (
+            ", " if pre_init_args else ""
+        )  # handle only kwargs and no regular args
+        pre_init_args += pre_init_kw_only_args
+
+    if pre_init and pre_init_has_args:
+        # If pre init method has arguments, pass same arguments as `__init__`
+        lines[0] = "self.__attrs_pre_init__(%s)" % pre_init_args
 
     return (
         "def %s(self, %s):\n    %s\n"

tests/test_dunders.py

@@ -6,6 +6,7 @@ Tests for dunder methods from `attrib._make`.
 
 
 import copy
+import inspect
 import pickle
 
 import pytest
@@ -84,10 +85,15 @@ def _add_init(cls, frozen):
     This function used to be part of _make.  It wasn't used anymore however
     the tests for it are still useful to test the behavior of _make_init.
     """
+    has_pre_init = bool(getattr(cls, "__attrs_pre_init__", False))
+
     cls.__init__ = _make_init(
         cls,
         cls.__attrs_attrs__,
-        getattr(cls, "__attrs_pre_init__", False),
+        has_pre_init,
+        len(inspect.signature(cls.__attrs_pre_init__).parameters) > 1
+        if has_pre_init
+        else False,
         getattr(cls, "__attrs_post_init__", False),
         frozen,
         _is_slot_cls(cls),

tests/test_make.py

@@ -613,21 +613,89 @@ class TestAttributes:
         assert C.D.__qualname__ == C.__qualname__ + ".D"
 
     @pytest.mark.parametrize("with_validation", [True, False])
-    def test_pre_init(self, with_validation, monkeypatch):
+    def test_pre_init(self, with_validation):
         """
         Verify that __attrs_pre_init__ gets called if defined.
         """
-        monkeypatch.setattr(_config, "_run_validators", with_validation)
 
         @attr.s
         class C:
             def __attrs_pre_init__(self2):
                 self2.z = 30
 
-        c = C()
+        try:
+            attr.validators.set_disabled(not with_validation)
+            c = C()
+        finally:
+            attr.validators.set_disabled(False)
 
         assert 30 == getattr(c, "z", None)
 
+    @pytest.mark.parametrize("with_validation", [True, False])
+    def test_pre_init_args(self, with_validation):
+        """
+        Verify that __attrs_pre_init__ gets called with extra args if defined.
+        """
+
+        @attr.s
+        class C:
+            x = attr.ib()
+
+            def __attrs_pre_init__(self2, x):
+                self2.z = x + 1
+
+        try:
+            attr.validators.set_disabled(not with_validation)
+            c = C(x=10)
+        finally:
+            attr.validators.set_disabled(False)
+
+        assert 11 == getattr(c, "z", None)
+
+    @pytest.mark.parametrize("with_validation", [True, False])
+    def test_pre_init_kwargs(self, with_validation):
+        """
+        Verify that __attrs_pre_init__ gets called with extra args and kwargs if defined.
+        """
+
+        @attr.s
+        class C:
+            x = attr.ib()
+            y = attr.field(kw_only=True)
+
+            def __attrs_pre_init__(self2, x, y):
+                self2.z = x + y + 1
+
+        try:
+            attr.validators.set_disabled(not with_validation)
+            c = C(10, y=11)
+        finally:
+            attr.validators.set_disabled(False)
+
+        assert 22 == getattr(c, "z", None)
+
+    @pytest.mark.parametrize("with_validation", [True, False])
+    def test_pre_init_kwargs_only(self, with_validation):
+        """
+        Verify that __attrs_pre_init__ gets called with extra kwargs only if
+        defined.
+        """
+
+        @attr.s
+        class C:
+            y = attr.field(kw_only=True)
+
+            def __attrs_pre_init__(self2, y):
+                self2.z = y + 1
+
+        try:
+            attr.validators.set_disabled(not with_validation)
+            c = C(y=11)
+        finally:
+            attr.validators.set_disabled(False)
+
+        assert 12 == getattr(c, "z", None)
+
     @pytest.mark.parametrize("with_validation", [True, False])
     def test_post_init(self, with_validation, monkeypatch):
         """