Use dict instead of OrderedDict on Python 3.7

OrderedDict is quite a bit heavier than just a dict.

src/_pytest/cacheprovider.py

@@ -6,7 +6,6 @@ ignores the external pytest-cache
 """
 import json
 import os
-from collections import OrderedDict
 from typing import Dict
 from typing import Generator
 from typing import List
@@ -23,6 +22,7 @@ from .pathlib import rm_rf
 from .reports import CollectReport
 from _pytest import nodes
 from _pytest._io import TerminalWriter
+from _pytest.compat import order_preserving_dict
 from _pytest.config import Config
 from _pytest.main import Session
 from _pytest.python import Module
@@ -338,8 +338,8 @@ class NFPlugin:
         self, session: Session, config: Config, items: List[nodes.Item]
     ) -> None:
         if self.active:
-            new_items = OrderedDict()  # type: OrderedDict[str, nodes.Item]
+            new_items = order_preserving_dict()  # type: Dict[str, nodes.Item]
-            other_items = OrderedDict()  # type: OrderedDict[str, nodes.Item]
+            other_items = order_preserving_dict()  # type: Dict[str, nodes.Item]
             for item in items:
                 if item.nodeid not in self.cached_nodeids:
                     new_items[item.nodeid] = item

src/_pytest/compat.py

@@ -381,3 +381,15 @@ else:
                 return self
             value = instance.__dict__[self.func.__name__] = self.func(instance)
             return value
+
+
+# Sometimes an algorithm needs a dict which yields items in the order in which
+# they were inserted when iterated. Since Python 3.7, `dict` preserves
+# insertion order. Since `dict` is faster and uses less memory than
+# `OrderedDict`, prefer to use it if possible.
+if sys.version_info >= (3, 7):
+    order_preserving_dict = dict
+else:
+    from collections import OrderedDict
+
+    order_preserving_dict = OrderedDict

src/_pytest/fixtures.py

@@ -4,7 +4,6 @@ import sys
 import warnings
 from collections import defaultdict
 from collections import deque
-from collections import OrderedDict
 from typing import Dict
 from typing import List
 from typing import Tuple
@@ -26,6 +25,7 @@ from _pytest.compat import getimfunc
 from _pytest.compat import getlocation
 from _pytest.compat import is_generator
 from _pytest.compat import NOTSET
+from _pytest.compat import order_preserving_dict
 from _pytest.compat import safe_getattr
 from _pytest.compat import TYPE_CHECKING
 from _pytest.deprecated import FILLFUNCARGS
@@ -220,12 +220,14 @@ def reorder_items(items):
         argkeys_cache[scopenum] = d = {}
         items_by_argkey[scopenum] = item_d = defaultdict(deque)
         for item in items:
-            keys = OrderedDict.fromkeys(get_parametrized_fixture_keys(item, scopenum))
+            keys = order_preserving_dict.fromkeys(
+                get_parametrized_fixture_keys(item, scopenum)
+            )
             if keys:
                 d[item] = keys
                 for key in keys:
                     item_d[key].append(item)
-    items = OrderedDict.fromkeys(items)
+    items = order_preserving_dict.fromkeys(items)
     return list(reorder_items_atscope(items, argkeys_cache, items_by_argkey, 0))
 
 
@@ -240,17 +242,17 @@ def reorder_items_atscope(items, argkeys_cache, items_by_argkey, scopenum):
         return items
     ignore = set()
     items_deque = deque(items)
-    items_done = OrderedDict()
+    items_done = order_preserving_dict()
     scoped_items_by_argkey = items_by_argkey[scopenum]
     scoped_argkeys_cache = argkeys_cache[scopenum]
     while items_deque:
-        no_argkey_group = OrderedDict()
+        no_argkey_group = order_preserving_dict()
         slicing_argkey = None
         while items_deque:
             item = items_deque.popleft()
             if item in items_done or item in no_argkey_group:
                 continue
-            argkeys = OrderedDict.fromkeys(
+            argkeys = order_preserving_dict.fromkeys(
                 k for k in scoped_argkeys_cache.get(item, []) if k not in ignore
             )
             if not argkeys:

src/_pytest/terminal.py

@@ -3,7 +3,6 @@
 This is a good source for looking at the various reporting hooks.
 """
 import argparse
-import collections
 import datetime
 import inspect
 import platform
@@ -28,6 +27,7 @@ from more_itertools import collapse
 import pytest
 from _pytest import nodes
 from _pytest._io import TerminalWriter
+from _pytest.compat import order_preserving_dict
 from _pytest.config import Config
 from _pytest.config import ExitCode
 from _pytest.deprecated import TERMINALWRITER_WRITER
@@ -839,8 +839,8 @@ class TerminalReporter:
                 return
 
             reports_grouped_by_message = (
-                collections.OrderedDict()
+                order_preserving_dict()
-            )  # type: collections.OrderedDict[str, List[WarningReport]]
+            )  # type: Dict[str, List[WarningReport]]
             for wr in warning_reports:
                 reports_grouped_by_message.setdefault(wr.message, []).append(wr)
 
@@ -854,9 +854,7 @@ class TerminalReporter:
                 if len(locations) < 10:
                     return "\n".join(map(str, locations))
 
-                counts_by_filename = (
+                counts_by_filename = order_preserving_dict()  # type: Dict[str, int]
-                    collections.OrderedDict()
-                )  # type: collections.OrderedDict[str, int]
                 for loc in locations:
                     key = str(loc).split("::", 1)[0]
                     counts_by_filename[key] = counts_by_filename.get(key, 0) + 1