Add support for RATIO, python Fraction types

If available, the `fractions` python module is imported and used
to handle RATIO types. If the module is not available (< 2.6) then
the ratio will be converted to a float.

Fixes issue bendudson#26

README.org

@@ -113,23 +113,23 @@ Data is passed between python and lisp as text. The python function
 reader; the lisp function =pythonize= outputs strings which can be
 =eval='d in python. The following type conversions are done:
 
-| Lisp type | Python type   |
-|-----------+---------------|
-| NIL       | None          |
-| integer   | int           |
-| ratio     | float         |
-| real      | float         |
-| complex   | complex float |
-| string    | str           |
-| hash map  | dict          |
-| list      | tuple         |
-| vector    | list          |
-| array     | NumPy array   |
-| symbol    | Symbol class  |
-| function  | function      |
+| Lisp type | Python type        |
+|-----------+--------------------|
+| NIL       | None               |
+| integer   | int                |
+| ratio     | fractions.Fraction |
+| real      | float              |
+| complex   | complex float      |
+| string    | str                |
+| hash map  | dict               |
+| list      | tuple              |
+| vector    | list               |
+| array     | NumPy array        |
+| symbol    | Symbol class       |
+| function  | function           |
 
 Note that python does not have all the numerical types which lisp has,
-for example rational numbers or complex integers.
+for example complex integers.
 
 Because =python-eval= and =python-exec= evaluate strings as python
 expressions, strings passed to them are not escaped or converted as

py4cl.py

@@ -475,6 +475,19 @@ def message_dispatch_loop():
 except:
     pass
 
+# Handle fractions (RATIO type)
+# Lisp will pass strings containing "_py4cl_fraction(n,d)"
+# where n and d are integers.
+try:
+    import fractions
+    eval_globals["_py4cl_fraction"] = fractions.Fraction
+
+    # Turn a Fraction into a Lisp RATIO
+    lispifiers[fractions.Fraction] = str
+except:
+    # In python2, ensure that fractions are converted to floats
+    eval_globals["_py4cl_fraction"] = lambda a,b : float(a)/b
+
 async_results = {}  # Store for function results. Might be Exception
 async_handle = itertools.count(0) # Running counter
 

src/writer.lisp

@@ -155,6 +155,15 @@ The lisp function is stored in the same object store as other objects."
                (write-to-string (python-object-handle obj))
                "]"))
 
+(defmethod pythonize ((obj ratio))
+  "Handle ratios, using Python's Fraction if available"
+  (concatenate 'string
+               "_py4cl_fraction("
+               (pythonize (numerator obj))
+               ","
+               (pythonize (denominator obj))
+               ")"))
+
 (defun stream-write-string (str stream)
   "Write a string to a stream, putting the length first"
   ;; Convert the value to a string

tests/tests.lisp

@@ -692,3 +692,20 @@ class Foo():
             ;; Check if no "residue" left
 
             (assert-equalp 5 (py4cl:python-eval 5)))))
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;; Rational numbers
+
+(deftest ratios (pytests)
+  ;; Ratios survive a round trip
+  (assert-equalp 1/2
+      (py4cl:python-eval 1/2))
+
+  ;; Ratios (Fractions in python) can be manipulated
+  (assert-equalp 1/4
+      (py4cl:python-eval 1/2 "/" 2))
+
+  ;; Complex ratios not supported in python so converts to floats
+  (assert-equality #'= #C(0.5 1.0)
+    (py4cl:python-eval #C(1 2) "*" 1/2)))
+