New restriction on pow(x, y, z): If z is not None, x and y must be of

integer types, and y must be >= 0.  See discussion at
http://sf.net/tracker/index.php?func=detail&aid=457066&group_id=5470&atid=105470

Doc/lib/libfuncs.tex

@@ -519,8 +519,14 @@ the interpreter.
   case, all arguments are converted to float and a float result is
   delivered.  For example, \code{10**2} returns \code{100}, but
   \code{10**-2} returns \code{0.01}.  (This last feature was added in
-  Python 2.2.  In Python 2.1 and before, a negative second argument
-  would raise an exception.)
+  Python 2.2.  In Python 2.1 and before, if both arguments were of integer
+  types and the second argument was negative, an exception was raised.)
+  If the second argument is negative, the third argument must be omitted. 
+  If \var{z} is present, \var{x} and \var{y} must be of integer types,
+  and \var{y} must be non-negative.  (This restriction was added in
+  Python 2.2.  In Python 2.1 and before, floating 3-argument \code{pow()}
+  returned platform-dependent results depending on floating-point
+  rounding accidents.)
 \end{funcdesc}
 
 \begin{funcdesc}{range}{\optional{start,} stop\optional{, step}}

Lib/test/output/test_pow

@@ -22,8 +22,4 @@ The number in both columns should match.
 -3L -3L
 -7L -7L
 
-3.0 3.0
--5.0 -5.0
--1.0 -1.0
--7.0 -7.0
 

Lib/test/test_b2.py

@@ -82,15 +82,26 @@ if fcmp(pow(2.,10), 1024.): raise TestFailed, 'pow(2.,10)'
 if fcmp(pow(2.,20), 1024.*1024.): raise TestFailed, 'pow(2.,20)'
 if fcmp(pow(2.,30), 1024.*1024.*1024.): raise TestFailed, 'pow(2.,30)'
 #
-# XXX These don't work -- negative float to the float power...
-#if fcmp(pow(-2.,0), 1.): raise TestFailed, 'pow(-2.,0)'
-#if fcmp(pow(-2.,1), -2.): raise TestFailed, 'pow(-2.,1)'
-#if fcmp(pow(-2.,2), 4.): raise TestFailed, 'pow(-2.,2)'
-#if fcmp(pow(-2.,3), -8.): raise TestFailed, 'pow(-2.,3)'
-#
+if fcmp(pow(-2.,0), 1.): raise TestFailed, 'pow(-2.,0)'
+if fcmp(pow(-2.,1), -2.): raise TestFailed, 'pow(-2.,1)'
+if fcmp(pow(-2.,2), 4.): raise TestFailed, 'pow(-2.,2)'
+if fcmp(pow(-2.,3), -8.): raise TestFailed, 'pow(-2.,3)'
+
+from types import FloatType
 for x in 2, 2L, 2.0:
     for y in 10, 10L, 10.0:
         for z in 1000, 1000L, 1000.0:
+            if isinstance(x, FloatType) or \
+               isinstance(y, FloatType) or \
+               isinstance(z, FloatType):
+                try:
+                    pow(x, y, z)
+                except TypeError:
+                    pass
+                else:
+                    raise TestFailed("3-arg float pow() should have "
+                                     "raised TypeError %r" % (x, y, z))
+            else:
                 if fcmp(pow(x, y, z), 24.0):
                     raise TestFailed, 'pow(%s, %s, %s)' % (x, y, z)
 

Lib/test/test_long.py

@@ -314,10 +314,19 @@ def test_auto_overflow():
                 checkit(x, '**', y)
 
                 for z in special:
-                    if z != 0:
+                    if z != 0 :
+                        if y >= 0:
                             expected = pow(longx, longy, long(z))
                             got = pow(x, y, z)
                             checkit('pow', x, y, '%', z)
+                        else:
+                            try:
+                                pow(longx, longy, long(z))
+                            except TypeError:
+                                pass
+                            else:
+                                raise TestFailed("pow%r should have raised "
+                                "TypeError" % ((longx, longy, long(z))))
 
 # ---------------------------------------------------------------- do it
 

Lib/test/test_pow.py

@@ -64,6 +64,15 @@ def powtest(type):
         for j in range(jl, jh+1):
             for k in range(kl, kh+1):
                 if k != 0:
+                    if type == float or j < 0:
+                        try:
+                            pow(type(i),j,k)
+                        except TypeError:
+                            pass
+                        else:
+                            raise TestFailed("expected TypeError from "
+                                "pow%r" % ((type(i), j, k)))
+                        continue
                     if compare(pow(type(i),j,k), pow(type(i),j)% type(k)):
                         raise ValueError, "pow(" +str(i)+ "," +str(j)+ \
                              "," +str(k)+ ") != pow(" +str(i)+ "," + \
@@ -96,10 +105,6 @@ print `pow(-3L,3L) % -8`, `pow(-3L,3L,-8)`
 print `pow(5L,2) % -8`, `pow(5L,2,-8)`
 print
 
-print pow(3.0,3.0) % 8, pow(3.0,3.0,8)
-print pow(3.0,3.0) % -8, pow(3.0,3.0,-8)
-print pow(3.0,2) % -2, pow(3.0,2,-2)
-print pow(5.0,2) % -8, pow(5.0,2,-8)
 print
 
 for i in range(-10, 11):
@@ -112,8 +117,3 @@ for i in range(-10, 11):
             if j >= 0 and k != 0:
                 o = pow(long(i),j) % k
                 n = pow(long(i),j,k)
-                if o != n: print 'Long mismatch:', i,j,k
-            if i >= 0 and k != 0:
-                o = pow(float(i),j) % k
-                n = pow(float(i),j,k)
-                if o != n: print 'Float mismatch:', i,j,k

Misc/NEWS

@@ -3,6 +3,12 @@ What's New in Python 2.2a3?
 
 Core
 
+- The 3-argument builtin pow() no longer allows a third non-None argument
+  if either of the first two arguments is a float, or if both are of
+  integer types and the second argument is negative (in which latter case
+  the arguments are converted to float, so this is really the same
+  restriction).
+
 - The builtin dir() now returns more information, and sometimes much
   more, generally naming all attributes of an object, and all attributes
   reachable from the object via its class, and from its class's base

Objects/floatobject.c

@@ -492,11 +492,13 @@ static PyObject *
 float_pow(PyObject *v, PyObject *w, PyObject *z)
 {
 	double iv, iw, ix;
- /* XXX Doesn't handle overflows if z!=None yet; it may never do so :(
-  * The z parameter is really only going to be useful for integers and
-  * long integers.  Maybe something clever with logarithms could be done.
-  * [AMK]
-  */
+
+	if ((PyObject *)z != Py_None) {
+		PyErr_SetString(PyExc_TypeError,
+			"3rd argument to floating pow() must be None");
+		return NULL;
+	}
+
 	CONVERT_TO_DOUBLE(v, iv);
 	CONVERT_TO_DOUBLE(w, iw);
 
@@ -538,16 +540,6 @@ float_pow(PyObject *v, PyObject *w, PyObject *z)
 		PyErr_SetFromErrno(PyExc_OverflowError);
 		return NULL;
 	}
-	if ((PyObject *)z != Py_None) {
-		double iz;
-		CONVERT_TO_DOUBLE(z, iz);
-		PyFPE_START_PROTECT("pow", return 0)
-	 	ix = fmod(ix, iz);	/* XXX To Be Rewritten */
-	 	if (ix != 0 && ((iv < 0 && iz > 0) || (iv > 0 && iz < 0) )) {
-		     ix += iz;
-		}
-  		PyFPE_END_PROTECT(ix)
-	}
 	return PyFloat_FromDouble(ix);
 }
 

Objects/intobject.c

@@ -575,6 +575,11 @@ int_pow(PyIntObject *v, PyIntObject *w, PyIntObject *z)
 	CONVERT_TO_LONG(v, iv);
 	CONVERT_TO_LONG(w, iw);
 	if (iw < 0) {
+		if ((PyObject *)z != Py_None) {
+			PyErr_SetString(PyExc_TypeError, "integer pow() arg "
+			     "3 must not be specified when arg 2 is < 0");
+			return NULL;
+		}
 		/* Return a float.  This works because we know that
 		   this calls float_pow() which converts its
 		   arguments to double. */

Objects/longobject.c

@@ -1598,12 +1598,17 @@ long_pow(PyObject *v, PyObject *w, PyObject *x)
 	
 	size_b = b->ob_size;
 	if (size_b < 0) {
-		/* Return a float.  This works because we know that
-		   this calls float_pow() which converts its
-		   arguments to double. */
 		Py_DECREF(a);
 		Py_DECREF(b);
 		Py_DECREF(c);
+		if (x != Py_None) {
+			PyErr_SetString(PyExc_TypeError, "integer pow() arg "
+			     "3 must not be specified when arg 2 is < 0");
+			return NULL;
+		}
+		/* Return a float.  This works because we know that
+		   this calls float_pow() which converts its
+		   arguments to double. */
 		return PyFloat_Type.tp_as_number->nb_power(v, w, x);
 	}
 	z = (PyLongObject *)PyLong_FromLong(1L);