bpo-34002: Minor efficiency and clarity improvements in email package. (GH-7999)

* Check intersection of two sets explicitly

Comparing ``len(a) > ``len(a - b)`` is essentially looking for an
intersection between the two sets. If set ``b`` does not intersect ``a``
then ``len(a - b)`` will be equal to ``len(a)``. This logic is more
clearly expressed as ``a & b``.

* Change while/pop to a for-loop

Copying the list, then repeatedly popping the first element was
unnecessarily slow. I also cleaned up a couple other inefficiencies.
There's no need to unpack a tuple, then re-pack and append it. The list
can be created with the first element instead of empty. Secondly, the
``endswith`` method returns a bool, so there's no need for an if-
statement to set ``encoding`` to True or False.

* Use set.intersection to check for intersections

``a.intersection(b)`` method is more clear of purpose than ``not
a.isdisjoint(b)`` and avoids an unnecessary set construction that ``a &
set(b)`` performs.

* Use not isdisjoint instead of intersection

While it reads slightly worse, the isdisjoint method will stop when it
finds a counterexample and returns a bool, rather than looping over the
entire iterable and constructing a new set.

3 files changed, 13 insertions, 25 deletions

diff --git a/Lib/email/headerregistry.py b/Lib/email/headerregistry.py
index dcc960b..cc1d191 100644
--- a/Lib/email/headerregistry.py
+++ b/Lib/email/headerregistry.py
@@ -69,11 +69,9 @@ class Address:
         """The addr_spec (username@domain) portion of the address, quoted
         according to RFC 5322 rules, but with no Content Transfer Encoding.
         """
-        nameset = set(self.username)
-        if len(nameset) > len(nameset-parser.DOT_ATOM_ENDS):
-            lp = parser.quote_string(self.username)
-        else:
-            lp = self.username
+        lp = self.username
+        if not parser.DOT_ATOM_ENDS.isdisjoint(lp):
+            lp = parser.quote_string(lp)
         if self.domain:
             return lp + '@' + self.domain
         if not lp:
@@ -86,11 +84,9 @@ class Address:
                         self.display_name, self.username, self.domain)
 
     def __str__(self):
-        nameset = set(self.display_name)
-        if len(nameset) > len(nameset-parser.SPECIALS):
-            disp = parser.quote_string(self.display_name)
-        else:
-            disp = self.display_name
+        disp = self.display_name
+        if not parser.SPECIALS.isdisjoint(disp):
+            disp = parser.quote_string(disp)
         if disp:
             addr_spec = '' if self.addr_spec=='<>' else self.addr_spec
             return "{} <{}>".format(disp, addr_spec)
@@ -141,10 +137,8 @@ class Group:
         if self.display_name is None and len(self.addresses)==1:
             return str(self.addresses[0])
         disp = self.display_name
-        if disp is not None:
-            nameset = set(disp)
-            if len(nameset) > len(nameset-parser.SPECIALS):
-                disp = parser.quote_string(disp)
+        if disp is not None and not parser.SPECIALS.isdisjoint(disp):
+            disp = parser.quote_string(disp)
         adrstr = ", ".join(str(x) for x in self.addresses)
         adrstr = ' ' + adrstr if adrstr else adrstr
         return "{}:{};".format(disp, adrstr)
diff --git a/Lib/email/utils.py b/Lib/email/utils.py
index 858f620..b137ce3 100644
--- a/Lib/email/utils.py
+++ b/Lib/email/utils.py
@@ -259,21 +259,13 @@ def decode_params(params):
 
     params is a sequence of 2-tuples containing (param name, string value).
     """
-    # Copy params so we don't mess with the original
-    params = params[:]
-    new_params = []
+    new_params = [params[0]]
     # Map parameter's name to a list of continuations.  The values are a
     # 3-tuple of the continuation number, the string value, and a flag
     # specifying whether a particular segment is %-encoded.
     rfc2231_params = {}
-    name, value = params.pop(0)
-    new_params.append((name, value))
-    while params:
-        name, value = params.pop(0)
-        if name.endswith('*'):
-            encoded = True
-        else:
-            encoded = False
+    for name, value in params[1:]:
+        encoded = name.endswith('*')
         value = unquote(value)
         mo = rfc2231_continuation.match(name)
         if mo:
diff --git a/Misc/NEWS.d/next/Library/2019-09-19-19-58-33.bpo-34002.KBnaVX.rst b/Misc/NEWS.d/next/Library/2019-09-19-19-58-33.bpo-34002.KBnaVX.rst
new file mode 100644
index 0000000..5d4c231
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2019-09-19-19-58-33.bpo-34002.KBnaVX.rst
@@ -0,0 +1,2 @@
+Improve efficiency in parts of email package by changing while-pop to a for
+loop, using isdisjoint instead of set intersections.