Issue #5859: Remove '%f' to '%g' formatting switch for large floats.

Doc/library/stdtypes.rst

@@ -1321,9 +1321,9 @@ that ``'\0'`` is the end of the string.
 
 .. XXX Examples?
 
-For safety reasons, floating point precisions are clipped to 50; ``%f``
-conversions for numbers whose absolute value is over 1e50 are replaced by ``%g``
-conversions. [#]_  All other errors raise exceptions.
+.. versionchanged:: 3.1
+   ``%f`` conversions for numbers whose absolute value is over 1e50 are no
+   longer replaced by ``%g`` conversions.
 
 .. index::
    module: string
@@ -2723,10 +2723,6 @@ The following attributes are only supported by :term:`new-style class`\ es.
 .. [#] To format only a tuple you should therefore provide a singleton tuple whose only
    element is the tuple to be formatted.
 
-.. [#] These numbers are fairly arbitrary.  They are intended to avoid printing endless
-   strings of meaningless digits without hampering correct use and without having
-   to know the exact precision of floating point values on a particular machine.
-
 .. [#] The advantage of leaving the newline on is that returning an empty string is
    then an unambiguous EOF indication.  It is also possible (in cases where it
    might matter, for example, if you want to make an exact copy of a file while

Lib/test/formatfloat_testcases.txt

@@ -22,8 +22,8 @@
 %.0f 123.456 -> 123
 %.0f 1234.56 -> 1235
 %.0f 1e49 -> 9999999999999999464902769475481793196872414789632
--- %.0f 1e50 -> 100000000000000007629769841091887003294964970946560
 %.0f 9.9999999999999987e+49 -> 99999999999999986860582406952576489172979654066176
+%.0f 1e50 -> 100000000000000007629769841091887003294964970946560
 
 -- precision 1
 %.1f 0.0001 -> 0.0

Lib/test/test_types.py

@@ -538,10 +538,25 @@ def test(f, format_spec, result):
         test(-1.0, ' f', '-1.000000')
         test( 1.0, '+f', '+1.000000')
         test(-1.0, '+f', '-1.000000')
-        test(1.1234e90, 'f', '1.1234e+90')
-        test(1.1234e90, 'F', '1.1234e+90')
-        test(1.1234e200, 'f', '1.1234e+200')
-        test(1.1234e200, 'F', '1.1234e+200')
+
+        # Python versions <= 3.0 switched from 'f' to 'g' formatting for
+        # values larger than 1e50.  No longer.
+        f = 1.1234e90
+        for fmt in 'f', 'F':
+            # don't do a direct equality check, since on some
+            # platforms only the first few digits of dtoa
+            # will be reliable
+            result = f.__format__(fmt)
+            self.assertEqual(len(result), 98)
+            self.assertEqual(result[-7], '.')
+            self.assert_(result[:12] in ('112340000000', '112339999999'))
+        f = 1.1234e200
+        for fmt in 'f', 'F':
+            result = f.__format__(fmt)
+            self.assertEqual(len(result), 208)
+            self.assertEqual(result[-7], '.')
+            self.assert_(result[:12] in ('112340000000', '112339999999'))
+
 
         test( 1.0, 'e', '1.000000e+00')
         test(-1.0, 'e', '-1.000000e+00')

Misc/NEWS

@@ -12,8 +12,10 @@ 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 #5859: Remove switch from '%f' to '%g'-style formatting for
+  floats with absolute value over 1e50.  Also 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/stringlib/formatter.h

@@ -934,8 +934,6 @@ format_float_internal(PyObject *value,
 
     if (precision < 0)
         precision = 6;
-    if ((type == 'f' || type == 'F') && fabs(val) >= 1e50)
-        type = 'g';
 
     /* Cast "type", because if we're in unicode we need to pass a
        8-bit char. This is safe, because we've restricted what "type"

Objects/unicodeobject.c

@@ -8808,9 +8808,6 @@ formatfloat(PyObject *v, int flags, int prec, int type)
     if (prec < 0)
         prec = 6;
 
-    if (type == 'f' && fabs(x) >= 1e50)
-        type = 'g';
-
     p = PyOS_double_to_string(x, type, prec,
                               (flags & F_ALT) ? Py_DTSF_ALT : 0, NULL);
     if (p == NULL)