fix f-string formatting of '0'

Cython/Utility/TypeConversion.c

@@ -634,12 +634,6 @@ static CYTHON_INLINE PyObject* {{TO_PY_FUNCTION}}({{TYPE}} value, Py_ssize_t wid
         format_char = 'x';
     };
 
-    // single character unicode strings are cached in CPython => use PyUnicode_FromOrdinal() for them
-    if (unlikely((is_unsigned || value >= const_zero) && (width <= 1) && (
-            (format_char == 'o') ? value <= 7 : (format_char == 'd') ? value <= 9 : value <= 15))) {
-        return PyUnicode_FromOrdinal(hex_digits[((int) value)]);
-    }
-
     // surprise: even trivial sprintf() calls don't get optimised in gcc (4.8)
     remaining = value; /* not using abs(value) to avoid overflow problems */
     last_one_off = 0;
@@ -673,6 +667,8 @@ static CYTHON_INLINE PyObject* {{TO_PY_FUNCTION}}({{TYPE}} value, Py_ssize_t wid
     if (last_one_off) {
         assert(*dpos == '0');
         dpos++;
+    } else if (unlikely(dpos == end)) {
+        *(--dpos) = '0';
     }
     length = end - dpos;
     ulength = length;
@@ -689,6 +685,10 @@ static CYTHON_INLINE PyObject* {{TO_PY_FUNCTION}}({{TYPE}} value, Py_ssize_t wid
     if (width > ulength) {
         ulength = width;
     }
+    // single character unicode strings are cached in CPython => use PyUnicode_FromOrdinal() for them
+    if (ulength == 1) {
+        return PyUnicode_FromOrdinal(*dpos);
+    }
     return __Pyx_PyUnicode_BuildFromAscii(ulength, dpos, length, prepend_sign, padding_char);
 }
 

tests/run/fstring.pyx

@@ -67,6 +67,16 @@ def format_c_numbers(signed char c, short s, int n, long l, float f, double d):
     >>> print(s4)
     -C-14-3.14
 
+    >>> s1, s2, s3, s4 = format_c_numbers(0, 0, 0, 0, -2.3456, -0.1415926)
+    >>> print(s1)
+    0   000-2.35
+    >>> print(s2)
+    -0.142-2.3
+    >>> print(s3)
+       0f
+    >>> print(s4)
+    00000-0.142
+
     """
     s1 = f"{c}{s:4}{l}{n}{f:.3}"
     assert isinstance(s1, unicode), type(s1)
@@ -102,6 +112,34 @@ def format_c_numbers_max(int n, long l):
     return s1, s2
 
 
+def format_c_number_range(int n):
+    """
+    >>> for i in range(-1000, 1000):
+    ...     assert format_c_number_range(i) == str(i)
+    """
+    return f'{n}'
+
+
+def format_c_number_range_width(int n):
+    """
+    >>> for i in range(-1000, 1000):
+    ...     assert format_c_number_range_width(i) == '%04d' % i, format_c_number_range_width(i)
+    """
+    return f'{n:04}'
+
+
+def format_c_number_range_dyn_width(int n, int width):
+    """
+    >>> for i in range(-1000, 1000):
+    ...     assert format_c_number_range_dyn_width(i, 0) == str(i), format_c_number_range_dyn_width(i, 0)
+    ...     assert format_c_number_range_dyn_width(i, 1) == '%01d' % i, format_c_number_range_dyn_width(i, 1)
+    ...     assert format_c_number_range_dyn_width(i, 4) == '%04d' % i, format_c_number_range_dyn_width(i, 4)
+    ...     assert format_c_number_range_dyn_width(i, 5) == '%05d' % i, format_c_number_range_dyn_width(i, 5)
+    ...     assert format_c_number_range_dyn_width(i, 6) == '%06d' % i, format_c_number_range_dyn_width(i, 6)
+    """
+    return f'{n:0{width}}'
+
+
 def format_bool(bint x):
     """
     >>> a, b, c, d = format_bool(1)