FindPython: Add policy to manage lookup stratgey default.

Help/manual/cmake-policies.7.rst

@@ -57,6 +57,7 @@ Policies Introduced by CMake 3.15
 .. toctree::
    :maxdepth: 1
 
+   CMP0094: FindPython3, FindPython2 and FindPython use LOCATION for lookup strategy. </policy/CMP0094>
    CMP0093: FindBoost reports Boost_VERSION in x.y.z format. </policy/CMP0093>
    CMP0092: MSVC warning flags are not in CMAKE_{C,CXX}_FLAGS by default. </policy/CMP0092>
    CMP0091: MSVC runtime library flags are selected by an abstraction. </policy/CMP0091>

Help/policy/CMP0094.rst

@@ -0,0 +1,22 @@
+CMP0094
+-------
+
+Modules :module:`FindPython3`, :module:`FindPython2` and :module:`FindPython`
+use ``LOCATION`` for lookup strategy.
+
+Starting with CMake 3.15, Modules :module:`FindPython3`, :module:`FindPython2`
+and :module:`FindPython` set value ``LOCATION`` for, respectively, variables
+``Python3_FIND_STRATEGY``, ``Python2_FIND_STRATEGY`` and
+``Python_FIND_STRATEGY``. This policy provides compatibility with projects that
+expect the legacy behavior.
+
+The ``OLD`` behavior for this policy set value ``VERSION`` for variables
+``Python3_FIND_STRATEGY``, ``Python2_FIND_STRATEGY`` and
+``Python_FIND_STRATEGY``.
+
+This policy was introduced in CMake version 3.15.  Use the
+:command:`cmake_policy` command to set it to ``OLD`` or ``NEW`` explicitly.
+Unlike many policies, CMake version |release| does *not* warn
+when this policy is not set and simply uses the ``OLD`` behavior.
+
+.. include:: DEPRECATED.txt

Help/release/dev/FindPython-CMP0094.rst

@@ -0,0 +1,5 @@
+FindPython-CMP0094
+------------------
+
+* Modules :module:`FindPython3`, :module:`FindPython2` and :module:`FindPython`
+  have changed default strategy for lookup. See policy :policy:`CMP0094`.

Help/release/dev/FindPython-FIND_STRATEGY.rst

@@ -0,0 +1,5 @@
+FindPython-FIND_STRATEGY
+------------------------
+
+* Modules :module:`FindPython3`, :module:`FindPython2` and :module:`FindPython`
+  gain a new way to control lookup strategy.

Modules/FindPython.cmake

@@ -144,9 +144,11 @@ Hints
 
   * ``VERSION``: Try to find the most recent version in all specified
     locations.
-    This is the default.
+    This is the default if policy :policy:`CMP0094` is undefined or set to
+    ``OLD``.
   * ``LOCATION``: Stops lookup as soon as a version satisfying version
     constraints is founded.
+    This is the default if policy :policy:`CMP0094` is set to ``NEW``.
 
 ``Python_FIND_REGISTRY``
   On Windows the ``Python_FIND_REGISTRY`` variable determine the order

Modules/FindPython/Support.cmake

@@ -5,8 +5,14 @@
 # This file is a "template" file used by various FindPython modules.
 #
 
+cmake_policy (GET CMP0094 _${_PYTHON_PREFIX}_LOOKUP_POLICY)
+
 cmake_policy (VERSION 3.7)
 
+if (_${_PYTHON_PREFIX}_LOOKUP_POLICY)
+  cmake_policy (SET CMP0094 ${_${_PYTHON_PREFIX}_LOOKUP_POLICY})
+endif()
+
 #
 # Initial configuration
 #
@@ -316,13 +322,17 @@ if (${_PYTHON_PREFIX}_FIND_VERSION_COUNT GREATER 1)
 endif()
 
 # Define lookup strategy
-set (_${_PYTHON_PREFIX}_FIND_STRATEGY "VERSION")
+if (_${_PYTHON_PREFIX}_LOOKUP_POLICY STREQUAL "NEW")
+  set (_${_PYTHON_PREFIX}_FIND_STRATEGY "LOCATION")
+else()
+  set (_${_PYTHON_PREFIX}_FIND_STRATEGY "VERSION")
+endif()
 if (DEFINED ${_PYTHON_PREFIX}_FIND_STRATEGY)
   if (NOT ${_PYTHON_PREFIX}_FIND_STRATEGY MATCHES "^(VERSION|LOCATION)$")
     message (AUTHOR_WARNING "Find${_PYTHON_PREFIX}: ${${_PYTHON_PREFIX}_FIND_STRATEGY}: invalid value for '${_PYTHON_PREFIX}_FIND_STRATEGY'. 'VERSION' or 'LOCATION' expected.")
     set (_${_PYTHON_PREFIX}_FIND_STRATEGY "VERSION")
   else()
-  set (_${_PYTHON_PREFIX}_FIND_STRATEGY "${${_PYTHON_PREFIX}_FIND_STRATEGY}")
+    set (_${_PYTHON_PREFIX}_FIND_STRATEGY "${${_PYTHON_PREFIX}_FIND_STRATEGY}")
   endif()
 endif()
 

Modules/FindPython2.cmake

@@ -145,9 +145,11 @@ Hints
 
   * ``VERSION``: Try to find the most recent version in all specified
     locations.
-    This is the default.
+    This is the default if policy :policy:`CMP0094` is undefined or set to
+    ``OLD``.
   * ``LOCATION``: Stops lookup as soon as a version satisfying version
     constraints is founded.
+    This is the default if policy :policy:`CMP0094` is set to ``NEW``.
 
 ``Python2_FIND_REGISTRY``
   On Windows the ``Python2_FIND_REGISTRY`` variable determine the order

Modules/FindPython3.cmake

@@ -145,9 +145,11 @@ Hints
 
   * ``VERSION``: Try to find the most recent version in all specified
     locations.
-    This is the default.
+    This is the default if policy :policy:`CMP0094` is undefined or set to
+    ``OLD``.
   * ``LOCATION``: Stops lookup as soon as a version satisfying version
     constraints is founded.
+    This is the default if policy :policy:`CMP0094` is set to ``NEW``.
 
 ``Python3_FIND_REGISTRY``
   On Windows the ``Python3_FIND_REGISTRY`` variable determine the order

Source/cmPolicies.h

@@ -275,6 +275,10 @@ class cmMakefile;
          "MSVC warning flags are not in CMAKE_<LANG>_FLAGS by default.", 3,   \
          15, 0, cmPolicies::WARN)                                             \
   SELECT(POLICY, CMP0093, "FindBoost reports Boost_VERSION in x.y.z format.", \
+         3, 15, 0, cmPolicies::WARN)                                          \
+  SELECT(POLICY, CMP0094,                                                     \
+         "FindPython3,  FindPython2 and FindPyton use "                       \
+         "LOCATION for lookup strategy.",                                     \
          3, 15, 0, cmPolicies::WARN)
 
 #define CM_SELECT_ID(F, A1, A2, A3, A4, A5, A6) F(A1)