Prefer requirements with upper bounds

Author: notatallshaw

docs/html/topics/more-dependency-resolution.md

@@ -168,8 +168,11 @@ follows:
 * Prefer if any of the known requirements is "direct", e.g. points to an
     explicit URL.
 * If equal, prefer if any requirement is "pinned", i.e. contains
-    operator ``===`` or ``==``.
+    operator ``===`` or ``==`` without a wildcard.
+* Prefer if any requirement is "upper-bounded", operators that do
+    not allow all future versions, e.g. ``<``, ``<=``, ``~=``, and
+    ``==`` with a wildcard.
 * Order user-specified requirements by the order they are specified.
 * If equal, prefers "non-free" requirements, i.e. contains at least one
-    operator, such as ``>=`` or ``<``.
+    operator, such as ``>=`` or ``!=``.
 * If equal, order alphabetically for consistency (helps debuggability).

src/pip/_internal/resolution/resolvelib/provider.py

@@ -164,10 +164,13 @@ def get_preference(
         * Prefer if any of the known requirements is "direct", e.g. points to an
           explicit URL.
         * If equal, prefer if any requirement is "pinned", i.e. contains
-          operator ``===`` or ``==``.
+          operator ``===`` or ``==`` without a wildcard.
+        * Prefer if any requirement is "upper-bounded", operators that do
+          not allow all future versions, e.g. ``<``, ``<=``, ``~=``, and
+          ``==`` with a wildcard.
         * Order user-specified requirements by the order they are specified.
         * If equal, prefers "non-free" requirements, i.e. contains at least one
-          operator, such as ``>=`` or ``<``.
+          operator, such as ``>=`` or ``!=``.
         * If equal, order alphabetically for consistency (helps debuggability).
         """
         try:
@@ -193,12 +196,17 @@ def get_preference(
 
         direct = candidate is not None
         pinned = any(((op[:2] == "==") and ("*" not in ver)) for op, ver in operators)
+        upper_bounded = any(
+            ((op in ("<", "<=", "~=")) or (op == "==" and "*" in ver))
+            for op, ver in operators
+        )
         unfree = bool(operators)
         requested_order = self._user_requested.get(identifier, math.inf)
 
         return (
             not direct,
             not pinned,
+            not upper_bounded,
             requested_order,
             not unfree,
             identifier,

tests/unit/resolution_resolvelib/test_provider.py

@@ -42,47 +42,79 @@ def build_req_info(
             {"pinned-package": [build_req_info("pinned-package==1.0")]},
             [],
             {},
-            (False, False, math.inf, False, "pinned-package"),
+            (False, False, True, math.inf, False, "pinned-package"),
         ),
         # Star-specified package, i.e. with "*"
         (
             "star-specified-package",
             {"star-specified-package": [build_req_info("star-specified-package==1.*")]},
             [],
             {},
-            (False, True, math.inf, False, "star-specified-package"),
+            (False, True, False, math.inf, False, "star-specified-package"),
         ),
         # Package that caused backtracking
         (
             "backtrack-package",
             {"backtrack-package": [build_req_info("backtrack-package")]},
             [build_req_info("backtrack-package")],
             {},
-            (False, True, math.inf, True, "backtrack-package"),
+            (False, True, True, math.inf, True, "backtrack-package"),
         ),
         # Root package requested by user
         (
             "root-package",
             {"root-package": [build_req_info("root-package")]},
             [],
             {"root-package": 1},
-            (False, True, 1, True, "root-package"),
+            (False, True, True, 1, True, "root-package"),
         ),
         # Unfree package (with specifier operator)
         (
             "unfree-package",
-            {"unfree-package": [build_req_info("unfree-package<1")]},
+            {"unfree-package": [build_req_info("unfree-package!=1")]},
             [],
             {},
-            (False, True, math.inf, False, "unfree-package"),
+            (False, True, True, math.inf, False, "unfree-package"),
         ),
         # Free package (no operator)
         (
             "free-package",
             {"free-package": [build_req_info("free-package")]},
             [],
             {},
-            (False, True, math.inf, True, "free-package"),
+            (False, True, True, math.inf, True, "free-package"),
+        ),
+        # Upper bounded with <= operator
+        (
+            "upper-bound-lte-package",
+            {
+                "upper-bound-lte-package": [
+                    build_req_info("upper-bound-lte-package<=2.0")
+                ]
+            },
+            [],
+            {},
+            (False, True, False, math.inf, False, "upper-bound-lte-package"),
+        ),
+        # Upper bounded with ~= operator
+        (
+            "upper-bound-compatible-package",
+            {
+                "upper-bound-compatible-package": [
+                    build_req_info("upper-bound-compatible-package~=1.0")
+                ]
+            },
+            [],
+            {},
+            (False, True, False, math.inf, False, "upper-bound-compatible-package"),
+        ),
+        # Not upper bounded, using only >= operator
+        (
+            "lower-bound-package",
+            {"lower-bound-package": [build_req_info("lower-bound-package>=1.0")]},
+            [],
+            {},
+            (False, True, True, math.inf, False, "lower-bound-package"),
         ),
     ],
 )