Issue #5859: Remove use of fixed-length buffers for float formatting

in unicodeobject.c and the fallback version of PyOS_double_to_string.
As a result, operations like '%.120e' % 12.34 no longer raise an
exception.

Lib/test/string_tests.py

@@ -1105,14 +1105,7 @@ class MixinStrUnicodeUserStringTest:
             value = 0.01
             for x in range(60):
                 value = value * 3.141592655 / 3.0 * 10.0
-                # The formatfloat() code in stringobject.c and
-                # unicodeobject.c uses a 120 byte buffer and switches from
-                # 'f' formatting to 'g' at precision 50, so we expect
-                # OverflowErrors for the ranges x < 50 and prec >= 67.
-                if x < 50 and prec >= 67:
-                    self.checkraises(OverflowError, format, "__mod__", value)
-                else:
-                    self.checkcall(format, "__mod__", value)
+                self.checkcall(format, "__mod__", value)
 
     def test_inplace_rewrites(self):
         # Check that strings don't copy and modify cached single-character strings

Misc/NEWS

@@ -12,6 +12,9 @@ What's New in Python 3.1 beta 1?
 Core and Builtins
 -----------------
 
+- Issue #5859: Remove length restrictions for float formatting:
+  '%.67f' % 12.34 and '%.120e' % 12.34 no longer raise an exception.
+
 - Issue #1588: Add complex.__format__. For example, 
   format(complex(1, 2./3), '.5') now produces a sensible result.
 

Objects/unicodeobject.c

@@ -8792,73 +8792,30 @@ getnextarg(PyObject *args, Py_ssize_t arglen, Py_ssize_t *p_argidx)
     return NULL;
 }
 
-static void
-strtounicode(Py_UNICODE *buffer, const char *charbuffer, Py_ssize_t len)
-{
-    register Py_ssize_t i;
-    for (i = len - 1; i >= 0; i--)
-        buffer[i] = (Py_UNICODE) charbuffer[i];
-}
+/* Returns a new reference to a PyUnicode object, or NULL on failure. */
 
-static int
-formatfloat(Py_UNICODE *buf,
-            size_t buflen,
-            int flags,
-            int prec,
-            int type,
-            PyObject *v)
-{
-    /* eric.smith: To minimize disturbances in PyUnicode_Format (the
-       only caller of this routine), I'm going to keep the existing
-       API to this function. That means that we'll allocate memory and
-       then copy back into the supplied buffer. But that's better than
-       all of the changes that would be required in PyUnicode_Format
-       because it does lots of memory management tricks. */
-
-    char* p = NULL;
-    int result = -1;
+static PyObject *
+formatfloat(PyObject *v, int flags, int prec, int type)
+{
+    char *p;
+    PyObject *result;
     double x;
-    Py_ssize_t len;
 
     x = PyFloat_AsDouble(v);
     if (x == -1.0 && PyErr_Occurred())
-        goto done;
+        return NULL;
+
     if (prec < 0)
         prec = 6;
 
-    /* make sure that the decimal representation of precision really does
-       need at most 10 digits: platforms with sizeof(int) == 8 exist! */
-    if (prec > 0x7fffffffL) {
-        PyErr_SetString(PyExc_OverflowError,
-                        "outrageously large precision "
-                        "for formatted float");
-        goto done;
-    }
-
     if (type == 'f' && fabs(x) >= 1e50)
         type = 'g';
 
-    if (((type == 'g' || type == 'G') &&
-         buflen <= (size_t)10 + (size_t)prec) ||
-        ((type == 'f' || type == 'F') &&
-         buflen <= (size_t)53 + (size_t)prec)) {
-        PyErr_SetString(PyExc_OverflowError,
-                        "formatted float is too long (precision too large?)");
-        goto done;
-    }
-
     p = PyOS_double_to_string(x, type, prec,
                               (flags & F_ALT) ? Py_DTSF_ALT : 0, NULL);
-    len = strlen(p);
-    if (len+1 >= buflen) {
-        /* Caller supplied buffer is not large enough. */
-        PyErr_NoMemory();
-        goto done;
-    }
-    strtounicode(buf, p, len);
-    result = Py_SAFE_DOWNCAST(len, Py_ssize_t, int);
-
-done:
+    if (p == NULL)
+        return NULL;
+    result = PyUnicode_FromStringAndSize(p, strlen(p));
     PyMem_Free(p);
     return result;
 }
@@ -8940,14 +8897,9 @@ formatchar(Py_UNICODE *buf,
 }
 
 /* fmt%(v1,v2,...) is roughly equivalent to sprintf(fmt, v1, v2, ...)
-
-   FORMATBUFLEN is the length of the buffer in which the floats, ints, &
-   chars are formatted. XXX This is a magic number. Each formatting
-   routine does bounds checking to ensure no overflow, but a better
-   solution may be to malloc a buffer of appropriate size for each
-   format. For now, the current solution is sufficient.
+   FORMATBUFLEN is the length of the buffer in which chars are formatted.
 */
-#define FORMATBUFLEN (size_t)120
+#define FORMATBUFLEN (size_t)10
 
 PyObject *PyUnicode_Format(PyObject *format,
                            PyObject *args)
@@ -9012,7 +8964,7 @@ PyObject *PyUnicode_Format(PyObject *format,
             Py_UNICODE *pbuf;
             Py_UNICODE sign;
             Py_ssize_t len;
-            Py_UNICODE formatbuf[FORMATBUFLEN]; /* For format{float,int,char}() */
+            Py_UNICODE formatbuf[FORMATBUFLEN]; /* For formatchar() */
 
             fmt++;
             if (*fmt == '(') {
@@ -9257,11 +9209,11 @@ PyObject *PyUnicode_Format(PyObject *format,
             case 'F':
             case 'g':
             case 'G':
-                pbuf = formatbuf;
-                len = formatfloat(pbuf, sizeof(formatbuf)/sizeof(Py_UNICODE),
-                                  flags, prec, c, v);
-                if (len < 0)
+                temp = formatfloat(v, flags, prec, c);
+                if (!temp)
                     goto onError;
+                pbuf = PyUnicode_AS_UNICODE(temp);
+                len = PyUnicode_GET_SIZE(temp);
                 sign = 1;
                 if (flags & F_ZERO)
                     fill = '0';

Python/pystrtod.c

@@ -620,12 +620,10 @@ PyAPI_FUNC(char *) PyOS_double_to_string(double val,
                                          int flags,
                                          int *type)
 {
-	char buf[128];
 	char format[32];
-	Py_ssize_t len;
-	char *result;
-	char *p;
-	int t;
+	Py_ssize_t bufsize;
+	char *buf;
+	int t, exp;
 	int upper = 0;
 
 	/* Validate format_code, and map upper and lower case */
@@ -669,6 +667,61 @@ PyAPI_FUNC(char *) PyOS_double_to_string(double val,
 		return NULL;
 	}
 
+	/* Here's a quick-and-dirty calculation to figure out how big a buffer
+	   we need.  In general, for a finite float we need:
+
+	     1 byte for each digit of the decimal significand, and
+
+	     1 for a possible sign
+	     1 for a possible decimal point
+	     2 for a possible [eE][+-]
+	     1 for each digit of the exponent;  if we allow 19 digits
+	       total then we're safe up to exponents of 2**63.
+	     1 for the trailing nul byte
+
+	   This gives a total of 24 + the number of digits in the significand,
+	   and the number of digits in the significand is:
+
+	     for 'g' format: at most precision, except possibly
+	       when precision == 0, when it's 1.
+	     for 'e' format: precision+1
+	     for 'f' format: precision digits after the point, at least 1
+	       before.  To figure out how many digits appear before the point
+	       we have to examine the size of the number.  If fabs(val) < 1.0
+	       then there will be only one digit before the point.  If
+	       fabs(val) >= 1.0, then there are at most
+
+	         1+floor(log10(ceiling(fabs(val))))
+
+	       digits before the point (where the 'ceiling' allows for the
+	       possibility that the rounding rounds the integer part of val
+	       up).  A safe upper bound for the above quantity is
+	       1+floor(exp/3), where exp is the unique integer such that 0.5
+	       <= fabs(val)/2**exp < 1.0.  This exp can be obtained from
+	       frexp.
+
+	   So we allow room for precision+1 digits for all formats, plus an
+	   extra floor(exp/3) digits for 'f' format.
+
+	*/
+
+	if (Py_IS_NAN(val) || Py_IS_INFINITY(val))
+		/* 3 for 'inf'/'nan', 1 for sign, 1 for '\0' */
+		bufsize = 5;
+	else {
+		bufsize = 25 + precision;
+		if (format_code == 'f' && fabs(val) >= 1.0) {
+			frexp(val, &exp);
+			bufsize += exp/3;
+		}
+	}
+
+	buf = PyMem_Malloc(bufsize);
+	if (buf == NULL) {
+		PyErr_NoMemory();
+		return NULL;
+	}
+
 	/* Handle nan and inf. */
 	if (Py_IS_NAN(val)) {
 		strcpy(buf, "nan");
@@ -687,38 +740,29 @@ PyAPI_FUNC(char *) PyOS_double_to_string(double val,
 		PyOS_snprintf(format, sizeof(format), "%%%s.%i%c",
 			      (flags & Py_DTSF_ALT ? "#" : ""), precision,
 			      format_code);
-		_PyOS_ascii_formatd(buf, sizeof(buf), format, val, precision);
-	}
-
-	len = strlen(buf);
-
-	/* Add 1 for the trailing 0 byte.
-	   Add 1 because we might need to make room for the sign.
-	   */
-	result = PyMem_Malloc(len + 2);
-	if (result == NULL) {
-		PyErr_NoMemory();
-		return NULL;
+		_PyOS_ascii_formatd(buf, bufsize, format, val, precision);
 	}
-	p = result;
 
 	/* Add sign when requested.  It's convenient (esp. when formatting
 	 complex numbers) to include a sign even for inf and nan. */
-	if (flags & Py_DTSF_SIGN && buf[0] != '-')
-		*p++ = '+';
-
-	strcpy(p, buf);
-
+	if (flags & Py_DTSF_SIGN && buf[0] != '-') {
+		size_t len = strlen(buf);
+		/* the bufsize calculations above should ensure that we've got
+		   space to add a sign */
+		assert((size_t)bufsize >= len+2);
+		memmove(buf+1, buf, len+1);
+		buf[0] = '+';
+	}
 	if (upper) {
 		/* Convert to upper case. */
 		char *p1;
-		for (p1 = p; *p1; p1++)
+		for (p1 = buf; *p1; p1++)
 			*p1 = Py_TOUPPER(*p1);
 	}
 
 	if (type)
 		*type = t;
-	return result;
+	return buf;
 }
 
 #else