gh-115225: Parsing fractions correctly in datetime.time.fromisoformat

Lib/_pydatetime.py

@@ -377,6 +377,7 @@ def _parse_hh_mm_ss_ff(tstr):
 
     time_comps = [0, 0, 0, 0]
     pos = 0
+    comp = None
     for comp in range(0, 3):
         if (len_str - pos) < 2:
             raise ValueError("Incomplete time component")
@@ -392,6 +393,9 @@ def _parse_hh_mm_ss_ff(tstr):
         if not next_char or comp >= 2:
             break
 
+        if next_char == '.' or next_char == ',':
+            break
+
         if has_sep and next_char != ':':
             raise ValueError("Invalid time separator: %c" % next_char)
 
@@ -410,9 +414,14 @@ def _parse_hh_mm_ss_ff(tstr):
             else:
                 to_parse = len_remainder
 
-            time_comps[3] = int(tstr[pos:(pos+to_parse)])
-            if to_parse < 6:
-                time_comps[3] *= _FRACTION_CORRECTION[to_parse-1]
+            extra_time = float("." + tstr[pos:(pos+to_parse)])
+            while comp < 2:
+                element = "hour" if comp == 0 else "minute"
+                raise ValueError(f"Fractional {element} not supported")
+            if comp == 2:
+                time_comps[comp + 1] = int(extra_time * 1e6)
+
+            # parse the rest of characters
             if (len_remainder > to_parse
                     and not all(map(_is_ascii_digit, tstr[(pos+to_parse):]))):
                 raise ValueError("Non-digit values in unparsed fraction")

Misc/NEWS.d/next/Library/2024-02-11-20-56-52.gh-issue-115225.8uXOZy.rst

@@ -0,0 +1,2 @@
+ISO8601 Format recommends "a decimal fraction may be added to the lowest order time element present."
+Thus "T0314.15" should be valid and yield "03:14:09", but currently yields "03:14:00.150000". Due to deliberate choice, such an expression would now throw a ValueError because such a string is more likely a typo than an actual fractional value.

Modules/_datetimemodule.c

@@ -831,9 +831,11 @@ parse_hh_mm_ss_ff(const char *tstr, const char *tstr_end, int *hour,
     int *vals[3] = {hour, minute, second};
     // This is initialized to satisfy an erroneous compiler warning.
     unsigned char has_separator = 1;
+    unsigned char has_decimal = 0;
 
     // Parse [HH[:?MM[:?SS]]]
-    for (size_t i = 0; i < 3; ++i) {
+    size_t i = 0;
+    for (; i < 3; ++i) {
         p = parse_digits(p, vals[i], 2);
         if (NULL == p) {
             return -3;
@@ -844,42 +846,52 @@ parse_hh_mm_ss_ff(const char *tstr, const char *tstr_end, int *hour,
             has_separator = (c == ':');
         }
 
-        if (p >= p_end) {
+        if (p >= p_end) { // reached timezone info
             return c != '\0';
         }
         else if (has_separator && (c == ':')) {
             continue;
         }
         else if (c == '.' || c == ',') {
+            has_decimal = 1;
             break;
-        } else if (!has_separator) {
+        }
+        else if (!has_separator) {
             --p;
         } else {
             return -4;  // Malformed time separator
         }
     }
 
+    // expecting to get exhausted unless
+    if (!has_decimal)
+        return -3;
     // Parse fractional components
     size_t len_remains = p_end - p;
     size_t to_parse = len_remains;
     if (len_remains >= 6) {
         to_parse = 6;
     }
 
-    p = parse_digits(p, microsecond, to_parse);
+    int extra_time_digits = 0;
+    p = parse_digits(p, &extra_time_digits, to_parse);
     if (NULL == p) {
         return -3;
     }
 
-    static int correction[] = {
-        100000, 10000, 1000, 100, 10
+    int dividing_factor[] = {
+        10, 100, 1000, 10000, 100000, 1000000
     };
 
-    if (to_parse < 6) {
-        *microsecond *= correction[to_parse-1];
+    float extra_time = extra_time_digits / (dividing_factor[to_parse - 1] * 1.);
+    while (i < 2) {
+        return -3;
+    }
+    if (i == 2) {
+        *microsecond = (extra_time * dividing_factor[5]);
     }
 
-    while (is_digit(*p)){
+    while (is_digit(*p) && p < p_end) {
         ++p; // skip truncated digits
     }