py/formatfloat: Fix number of digits and exponent sign when rounding.

This patch fixes 2 things when printing a floating-point number that
requires rounding up of the mantissa:
- retain the correct precision; eg 0.99 becomes 1.0, not 1.00
- if the exponent goes from -1 to 0 then render it as +0, not -0

3 files changed, 5 insertions, 4 deletions

diff --git a/tests/float/string_format_modulo.py b/tests/float/string_format_modulo.py
index 03c8dd00a..aea534247 100644
--- a/tests/float/string_format_modulo.py
+++ b/tests/float/string_format_modulo.py
@@ -44,3 +44,6 @@ print(('%.40g' % 1e-3)[:2])
 print(('%.40g' % 1e-4)[:2])
 
 print("%.0g" % 1) # 0 precision 'g'
+
+print('%.1e' % 9.99) # round up with positive exponent
+print('%.1e' % 0.999) # round up with negative exponent
diff --git a/tests/float/string_format_modulo3.py b/tests/float/string_format_modulo3.py
index 563964786..5d26f2575 100644
--- a/tests/float/string_format_modulo3.py
+++ b/tests/float/string_format_modulo3.py
@@ -1,4 +1,3 @@
 # uPy and CPython outputs differ for the following
 print("%.1g" % -9.9) # round up 'g' with '-' sign
-print("%.1e" % 9.99) # round up with positive exponent
-print("%.1e" % 0.999) # round up with negative exponent
+print("%.2g" % 99.9) # round up
diff --git a/tests/float/string_format_modulo3.py.exp b/tests/float/string_format_modulo3.py.exp
index b158c7d7f..71432b340 100644
--- a/tests/float/string_format_modulo3.py.exp
+++ b/tests/float/string_format_modulo3.py.exp
@@ -1,3 +1,2 @@
 -10
-1.00e+01
-1.00e-00
+100