Output template now supports '0*d' zero padding

test/test_utils.py

@@ -33,6 +33,7 @@ from youtube_dl.utils import (
     ExtractorError,
     find_xpath_attr,
     fix_xml_ampersands,
+    auto_str_to_int_fmt,
     InAdvancePagedList,
     intlist_to_bytes,
     is_html,
@@ -985,5 +986,16 @@ The first line
         self.assertEqual(urshift(3, 1), 1)
         self.assertEqual(urshift(-3, 1), 2147483646)
 
+    def test_auto_str_to_int_fmt(self):
+        input = {"foo": "bar",
+                 "number": 1234,
+                 "s_number": "001234"}
+
+        self.assertEqual(auto_str_to_int_fmt("%(foo)s", input), "bar")
+        self.assertEqual(auto_str_to_int_fmt("%(number)d", input), "1234")
+        self.assertEqual(auto_str_to_int_fmt("%(s_number)d", input), "1234")
+        self.assertEqual(auto_str_to_int_fmt("%(s_number)07d", input), "0001234")
+        self.assertEqual(auto_str_to_int_fmt("%(foo)s%(number)d-%(s_number)d", input), "bar1234-1234")
+
 if __name__ == '__main__':
     unittest.main()

youtube_dl/YoutubeDL.py

@@ -41,6 +41,7 @@ from .compat import (
 from .utils import (
     age_restricted,
     args_to_str,
+    auto_str_to_int_fmt,
     ContentTooShortError,
     date_from_str,
     DateRange,
@@ -588,7 +589,7 @@ class YoutubeDL(object):
 
             outtmpl = self.params.get('outtmpl', DEFAULT_OUTTMPL)
             tmpl = compat_expanduser(outtmpl)
-            filename = tmpl % template_dict
+            filename = auto_str_to_int_fmt(tmpl, template_dict)
             # Temporary fix for #4787
             # 'Treat' all problem characters by passing filename through preferredencoding
             # to workaround encoding issues with subprocess on python2 @ Windows

youtube_dl/utils.py

@@ -2903,3 +2903,23 @@ def parse_m3u8_attributes(attrib):
 
 def urshift(val, n):
     return val >> n if val >= 0 else (val + 0x100000000) >> n
+
+
+def auto_str_to_int_fmt(fmt, input):
+    """
+    String formatting similar to '%' formatting, but supporting strings in the input for the 'd' conversion type.
+    """
+
+    regex_split = "(%\(\w+\)[0-9]*d)"
+    regex_grouped = "%\((\w+)\)[0-9]*d"
+    result = []
+
+    for s in re.split(regex_split, fmt):
+        match = re.match(regex_grouped, s)
+        if match is None:
+            result.append(s)
+        else:
+            mapping_key = match.group(1)
+            result.append(s % { mapping_key: int(input[mapping_key]) })
+
+    return "".join(result) % input