Allow subclasses to override _repr_latex_

The previous behavior was to completely ignore this hook once `init_printing` had been called, which was surprising to the user.

sympy/interactive/printing.py

@@ -249,58 +249,67 @@ def _result_display(self, arg):
     import IPython
     if V(IPython.__version__) >= '0.11':
 
-        printable_types = [Printable, float, tuple, list, set,
-                frozenset, dict, int]
+        # Printable is our own type, so we handle it with methods instead of
+        # the approach required by builtin types. This allows downstream
+        # packages to override the methods in their own classes.
+        printable_types = [float, tuple, list, set, frozenset, dict, int]
 
         plaintext_formatter = ip.display_formatter.formatters['text/plain']
 
-        for cls in printable_types:
+        # Exception to the rule above, IPython has better dispatching rules
+        # for plaintext printing, and we can't use `_repr_pretty_` without
+        # hitting a recursion error above.
+        for cls in printable_types + [Printable]:
             plaintext_formatter.for_type(cls, _print_plain)
 
         svg_formatter = ip.display_formatter.formatters['image/svg+xml']
         if use_latex in ('svg', ):
             debug("init_printing: using svg formatter")
             for cls in printable_types:
                 svg_formatter.for_type(cls, _print_latex_svg)
+            Printable._repr_svg_ = _print_latex_svg
         else:
             debug("init_printing: not using any svg formatter")
             for cls in printable_types:
                 # Better way to set this, but currently does not work in IPython
                 #png_formatter.for_type(cls, None)
                 if cls in svg_formatter.type_printers:
                     svg_formatter.type_printers.pop(cls)
+            Printable._repr_svg_ = None
 
         png_formatter = ip.display_formatter.formatters['image/png']
         if use_latex in (True, 'png'):
             debug("init_printing: using png formatter")
             for cls in printable_types:
                 png_formatter.for_type(cls, _print_latex_png)
+            Printable._repr_png_ = _print_latex_png
         elif use_latex == 'matplotlib':
             debug("init_printing: using matplotlib formatter")
             for cls in printable_types:
                 png_formatter.for_type(cls, _print_latex_matplotlib)
+            Printable._repr_png_ = _print_latex_matplotlib
         else:
             debug("init_printing: not using any png formatter")
             for cls in printable_types:
                 # Better way to set this, but currently does not work in IPython
                 #png_formatter.for_type(cls, None)
                 if cls in png_formatter.type_printers:
                     png_formatter.type_printers.pop(cls)
+            Printable._repr_png_ = None
 
         latex_formatter = ip.display_formatter.formatters['text/latex']
         if use_latex in (True, 'mathjax'):
             debug("init_printing: using mathjax formatter")
             for cls in printable_types:
                 latex_formatter.for_type(cls, _print_latex_text)
-            Printable._repr_latex_ = Printable._repr_latex_orig
+            Printable._repr_latex_ = _print_latex_text
         else:
             debug("init_printing: not using text/latex formatter")
             for cls in printable_types:
                 # Better way to set this, but currently does not work in IPython
                 #latex_formatter.for_type(cls, None)
                 if cls in latex_formatter.type_printers:
                     latex_formatter.type_printers.pop(cls)
-
             Printable._repr_latex_ = None
 
     else:

sympy/interactive/tests/test_ipythonprinting.py

@@ -190,3 +190,24 @@ def test_matplotlib_bad_latex():
     # issue 9799
     app.run_cell("from sympy import Piecewise, Symbol, Eq")
     app.run_cell("x = Symbol('x'); pw = format(Piecewise((1, Eq(x, 0)), (0, True)))")
+
+
+def test_override_repr_latex():
+    # Initialize and setup IPython session
+    app = init_ipython_session()
+    app.run_cell("import IPython")
+    app.run_cell("from sympy import init_printing")
+    app.run_cell("from sympy import Symbol")
+    app.run_cell("init_printing(use_latex=True)")
+    app.run_cell("""\
+    class SymbolWithOverload(Symbol):
+        def _repr_latex_(self):
+            return r"Hello " + super()._repr_latex_() + " world"
+    """)
+    app.run_cell("s = SymbolWithOverload('s')")
+
+    if int(ipython.__version__.split(".")[0]) < 1:
+        latex = app.user_ns['s']['text/latex']
+    else:
+        latex = app.user_ns['s'][0]['text/latex']
+    assert latex == r'Hello $\displaystyle s$ world'