Add __repr__ to requirement/candidate models

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

@@ -110,6 +110,13 @@ def __init__(
         self._version = version
         self._dist = None  # type: Optional[Distribution]
 
+    def __repr__(self):
+        # type: () -> str
+        return "{class_name}({link!r})".format(
+            class_name=self.__class__.__name__,
+            link=str(self.link),
+        )
+
     def __eq__(self, other):
         # type: (Any) -> bool
         if isinstance(other, self.__class__):
@@ -259,6 +266,13 @@ def __init__(
         skip_reason = "already satisfied"
         factory.preparer.prepare_installed_requirement(self._ireq, skip_reason)
 
+    def __repr__(self):
+        # type: () -> str
+        return "{class_name}({distribution!r})".format(
+            class_name=self.__class__.__name__,
+            distribution=self.dist,
+        )
+
     @property
     def name(self):
         # type: () -> str
@@ -314,6 +328,14 @@ def __init__(
         self.base = base
         self.extras = extras
 
+    def __repr__(self):
+        # type: () -> str
+        return "{class_name}(base={base!r}, extras={extras!r})".format(
+            class_name=self.__class__.__name__,
+            base=self.base,
+            extras=self.extras,
+        )
+
     @property
     def name(self):
         # type: () -> str

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

@@ -18,6 +18,13 @@ def __init__(self, candidate):
         # type: (Candidate) -> None
         self.candidate = candidate
 
+    def __repr__(self):
+        # type: () -> str
+        return "{class_name}({candidate!r})".format(
+            class_name=self.__class__.__name__,
+            candidate=self.candidate,
+        )
+
     @property
     def name(self):
         # type: () -> str
@@ -43,6 +50,13 @@ def __init__(self, name):
         # type: (str) -> None
         self._name = name
 
+    def __repr__(self):
+        # type: () -> str
+        return "{class_name}(name={name!r})".format(
+            class_name=self.__class__.__name__,
+            name=self._name,
+        )
+
     @property
     def name(self):
         # type: () -> str
@@ -65,6 +79,13 @@ def __init__(self, ireq, factory):
         self._factory = factory
         self.extras = ireq.req.extras
 
+    def __repr__(self):
+        # type: () -> str
+        return "{class_name}({requirement!r})".format(
+            class_name=self.__class__.__name__,
+            requirement=str(self._ireq.req),
+        )
+
     @property
     def name(self):
         # type: () -> str