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

6 files changed, 27 insertions, 19 deletions

diff --git a/Doc/library/stdtypes.rst b/Doc/library/stdtypes.rst
index 72e2fb44b7..c8c2c801ae 100644
--- a/Doc/library/stdtypes.rst
+++ b/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
diff --git a/Lib/test/formatfloat_testcases.txt b/Lib/test/formatfloat_testcases.txt
index 287019f929..4cf20aa8c9 100644
--- a/Lib/test/formatfloat_testcases.txt
+++ b/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
diff --git a/Lib/test/test_types.py b/Lib/test/test_types.py
index 230b102342..b85d337481 100644
--- a/Lib/test/test_types.py
+++ b/Lib/test/test_types.py
@@ -538,10 +538,25 @@ class TypesTests(unittest.TestCase):
         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')
diff --git a/Misc/NEWS b/Misc/NEWS
index 3e8f5e6478..d6ea20999f 100644
--- a/Misc/NEWS
+++ b/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.
diff --git a/Objects/stringlib/formatter.h b/Objects/stringlib/formatter.h
index c5cda4cebf..d9d47b9f49 100644
--- a/Objects/stringlib/formatter.h
+++ b/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"
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 31b9a73683..68d4fc41ae 100644
--- a/Objects/unicodeobject.c
+++ b/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)