diff --git a/src/sage/misc/cachefunc.pyx b/src/sage/misc/cachefunc.pyx
index af341a2c849..a5ad98e0364 100644
--- a/src/sage/misc/cachefunc.pyx
+++ b/src/sage/misc/cachefunc.pyx
@@ -420,8 +420,7 @@ cdef extern from "methodobject.h":
     cdef int PyCFunction_GetFlags(object op) except -1
 
 import os
-from os.path import relpath, normpath, commonprefix
-from sage.misc.sageinspect import sage_getfile, sage_getsourcelines, sage_getargspec
+from sage.misc.sageinspect import sage_getfile_relative, sage_getsourcelines, sage_getargspec
 from inspect import isfunction
 
 from sage.misc.weak_dict cimport CachedWeakValueDictionary
@@ -875,24 +874,7 @@ cdef class CachedFunction():
         if not doc or _extract_embedded_position(doc) is None:
             try:
                 sourcelines = sage_getsourcelines(f)
-                filename = sage_getfile(f)
-
-                def directories():
-                    try:
-                        from sage.env import SAGE_SRC
-                    except ImportError:
-                        pass
-                    else:
-                        if SAGE_SRC:
-                            yield normpath(os.path.join(SAGE_SRC, 'sage'))
-                    import sage
-                    yield from sage.__path__
-
-                for directory in directories():
-                    if commonprefix([filename, directory]) == directory:
-                        filename = os.path.join('sage', relpath(filename, directory))
-                        break
-
+                filename = sage_getfile_relative(f)
                 #this is a rather expensive way of getting the line number, because
                 #retrieving the source requires reading the source file and in many
                 #cases this is not required (in cython it's embedded in the docstring,
diff --git a/src/sage/misc/sageinspect.py b/src/sage/misc/sageinspect.py
index 4930601efad..f09a3a225ae 100644
--- a/src/sage/misc/sageinspect.py
+++ b/src/sage/misc/sageinspect.py
@@ -1413,6 +1413,52 @@ def sage_getfile(obj):
     return sourcefile
 
 
+def sage_getfile_relative(obj):
+    r"""
+    Get the file name associated to ``obj`` as a string.
+
+    This is the same as :func:`sage_getfile`, but
+    if the source file is part of the ``sage.*`` namespace, it
+    makes the file name relative so that it starts with ``sage/``.
+
+    INPUT: ``obj``, a Sage object, module, etc.
+
+    EXAMPLES::
+
+        sage: from sage.misc.sageinspect import sage_getfile_relative
+        sage: sage_getfile_relative(sage.rings.rational)
+        'sage/rings/rational.pyx'
+        sage: sage_getfile_relative(Sq)
+        'sage/algebras/steenrod/steenrod_algebra.py'
+        sage: sage_getfile_relative(x)
+        'sage/symbolic/expression.pyx'
+        sage: sage_getfile_relative(range)
+        ''
+    """
+    filename = sage_getfile(obj)
+    if not filename:
+        return filename
+
+    from os.path import relpath, normpath, commonprefix
+
+    def directories():
+        try:
+            from sage.env import SAGE_SRC
+        except ImportError:
+            pass
+        else:
+            if SAGE_SRC:
+                yield normpath(os.path.join(SAGE_SRC, 'sage'))
+        import sage
+        yield from sage.__path__
+
+    for directory in directories():
+        if commonprefix([filename, directory]) == directory:
+            return os.path.join('sage', relpath(filename, directory))
+
+    return filename
+
+
 def sage_getargspec(obj):
     r"""
     Return the names and default values of a function's arguments.
diff --git a/src/sage/sets/set_from_iterator.py b/src/sage/sets/set_from_iterator.py
index af32a3be56a..735ec7d3448 100644
--- a/src/sage/sets/set_from_iterator.py
+++ b/src/sage/sets/set_from_iterator.py
@@ -464,37 +464,15 @@ def _instancedoc_(self):
                Calls the PARI "isprime" function.
         """
         # Duplicates sage.misc.cachefunc.CachedFunction._instancedoc_
-        from sage.misc.sageinspect import sage_getsourcelines, sage_getfile, _extract_embedded_position
+        from sage.misc.sageinspect import sage_getsourcelines, sage_getfile_relative, _extract_embedded_position
         f = self.f
         doc = f.__doc__ or ''
         if _extract_embedded_position(doc) is None:
             try:
-                from os.path import normpath, commonprefix
                 sourcelines = sage_getsourcelines(f)
-                filename = sage_getfile(f)
-
-                # The following is a heuristics to get
-                # the file name of the cached function
-                # or method
-
-                def directories():
-                    try:
-                        from sage.env import SAGE_SRC
-                    except ImportError:
-                        pass
-                    else:
-                        if SAGE_SRC:
-                            yield normpath(os.path.join(SAGE_SRC, 'sage'))
-                    import sage
-                    yield from sage.__path__
-
-                for directory in directories():
-                    if commonprefix([filename, directory]) == directory:
-                        filename = os.path.join('sage', relpath(filename, directory))
-                        break
-
-                file_info = "File: %s (starting at line %d)\n"%(filename,sourcelines[1])
-                doc = file_info+doc
+                filename = sage_getfile_relative(f)
+                file_info = "File: %s (starting at line %d)\n" % (filename, sourcelines[1])
+                doc = file_info + doc
             except IOError:
                 pass
         return doc