gh-69142: add %:z strftime format code (gh-95983)

datetime.isoformat generates the tzoffset with colons, but there
was no format code to make strftime output the same format.

for simplicity and consistency the %:z formatting behaves mostly
as %z, with the exception of adding colons. this includes the
dynamic behaviour of adding seconds and microseconds only when
needed (when not 0).

this fixes the still open "generate" part of this issue:

https://github.com/python/cpython/issues/69142

Co-authored-by: Kumar Aditya <59607654+kumaraditya303@users.noreply.github.com>

1 files changed, 24 insertions, 21 deletions

diff --git a/Lib/datetime.py b/Lib/datetime.py
index 00ded32..007114a 100644
--- a/Lib/datetime.py
+++ b/Lib/datetime.py
@@ -179,7 +179,7 @@ def _format_time(hh, mm, ss, us, timespec='auto'):
     else:
         return fmt.format(hh, mm, ss, us)
 
-def _format_offset(off):
+def _format_offset(off, sep=':'):
     s = ''
     if off is not None:
         if off.days < 0:
@@ -189,9 +189,9 @@ def _format_offset(off):
             sign = "+"
         hh, mm = divmod(off, timedelta(hours=1))
         mm, ss = divmod(mm, timedelta(minutes=1))
-        s += "%s%02d:%02d" % (sign, hh, mm)
+        s += "%s%02d%s%02d" % (sign, hh, sep, mm)
         if ss or ss.microseconds:
-            s += ":%02d" % ss.seconds
+            s += "%s%02d" % (sep, ss.seconds)
 
             if ss.microseconds:
                 s += '.%06d' % ss.microseconds
@@ -202,9 +202,10 @@ def _wrap_strftime(object, format, timetuple):
     # Don't call utcoffset() or tzname() unless actually needed.
     freplace = None  # the string to use for %f
     zreplace = None  # the string to use for %z
+    colonzreplace = None  # the string to use for %:z
     Zreplace = None  # the string to use for %Z
 
-    # Scan format for %z and %Z escapes, replacing as needed.
+    # Scan format for %z, %:z and %Z escapes, replacing as needed.
     newformat = []
     push = newformat.append
     i, n = 0, len(format)
@@ -222,26 +223,28 @@ def _wrap_strftime(object, format, timetuple):
                     newformat.append(freplace)
                 elif ch == 'z':
                     if zreplace is None:
-                        zreplace = ""
                         if hasattr(object, "utcoffset"):
-                            offset = object.utcoffset()
-                            if offset is not None:
-                                sign = '+'
-                                if offset.days < 0:
-                                    offset = -offset
-                                    sign = '-'
-                                h, rest = divmod(offset, timedelta(hours=1))
-                                m, rest = divmod(rest, timedelta(minutes=1))
-                                s = rest.seconds
-                                u = offset.microseconds
-                                if u:
-                                    zreplace = '%c%02d%02d%02d.%06d' % (sign, h, m, s, u)
-                                elif s:
-                                    zreplace = '%c%02d%02d%02d' % (sign, h, m, s)
-                                else:
-                                    zreplace = '%c%02d%02d' % (sign, h, m)
+                            zreplace = _format_offset(object.utcoffset(), sep="")
+                        else:
+                            zreplace = ""
                     assert '%' not in zreplace
                     newformat.append(zreplace)
+                elif ch == ':':
+                    if i < n:
+                        ch2 = format[i]
+                        i += 1
+                        if ch2 == 'z':
+                            if colonzreplace is None:
+                                if hasattr(object, "utcoffset"):
+                                    colonzreplace = _format_offset(object.utcoffset(), sep=":")
+                                else:
+                                    colonzreplace = ""
+                            assert '%' not in colonzreplace
+                            newformat.append(colonzreplace)
+                        else:
+                            push('%')
+                            push(ch)
+                            push(ch2)
                 elif ch == 'Z':
                     if Zreplace is None:
                         Zreplace = ""