bpo-39939: Add str.removeprefix and str.removesuffix (GH-18939)

Added str.removeprefix and str.removesuffix methods and corresponding
bytes, bytearray, and collections.UserString methods to remove affixes
from a string if present. See PEP 616 for a full description.

3 files changed, 45 insertions, 1 deletions

diff --git a/Lib/collections/__init__.py b/Lib/collections/__init__.py
index e198406..bb9a605 100644
--- a/Lib/collections/__init__.py
+++ b/Lib/collections/__init__.py
@@ -1239,6 +1239,14 @@ class UserString(_collections_abc.Sequence):
         if isinstance(sub, UserString):
             sub = sub.data
         return self.data.count(sub, start, end)
+    def removeprefix(self, prefix, /):
+        if isinstance(prefix, UserString):
+            prefix = prefix.data
+        return self.__class__(self.data.removeprefix(prefix))
+    def removesuffix(self, suffix, /):
+        if isinstance(suffix, UserString):
+            suffix = suffix.data
+        return self.__class__(self.data.removesuffix(suffix))
     def encode(self, encoding='utf-8', errors='strict'):
         encoding = 'utf-8' if encoding is None else encoding
         errors = 'strict' if errors is None else errors
diff --git a/Lib/test/string_tests.py b/Lib/test/string_tests.py
index 948e2a3..527f505 100644
--- a/Lib/test/string_tests.py
+++ b/Lib/test/string_tests.py
@@ -682,6 +682,42 @@ class BaseTest:
         self.checkraises(OverflowError, A2_16, "replace", "A", A2_16)
         self.checkraises(OverflowError, A2_16, "replace", "AA", A2_16+A2_16)
 
+    def test_removeprefix(self):
+        self.checkequal('am', 'spam', 'removeprefix', 'sp')
+        self.checkequal('spamspam', 'spamspamspam', 'removeprefix', 'spam')
+        self.checkequal('spam', 'spam', 'removeprefix', 'python')
+        self.checkequal('spam', 'spam', 'removeprefix', 'spider')
+        self.checkequal('spam', 'spam', 'removeprefix', 'spam and eggs')
+
+        self.checkequal('', '', 'removeprefix', '')
+        self.checkequal('', '', 'removeprefix', 'abcde')
+        self.checkequal('abcde', 'abcde', 'removeprefix', '')
+        self.checkequal('', 'abcde', 'removeprefix', 'abcde')
+
+        self.checkraises(TypeError, 'hello', 'removeprefix')
+        self.checkraises(TypeError, 'hello', 'removeprefix', 42)
+        self.checkraises(TypeError, 'hello', 'removeprefix', 42, 'h')
+        self.checkraises(TypeError, 'hello', 'removeprefix', 'h', 42)
+        self.checkraises(TypeError, 'hello', 'removeprefix', ("he", "l"))
+
+    def test_removesuffix(self):
+        self.checkequal('sp', 'spam', 'removesuffix', 'am')
+        self.checkequal('spamspam', 'spamspamspam', 'removesuffix', 'spam')
+        self.checkequal('spam', 'spam', 'removesuffix', 'python')
+        self.checkequal('spam', 'spam', 'removesuffix', 'blam')
+        self.checkequal('spam', 'spam', 'removesuffix', 'eggs and spam')
+
+        self.checkequal('', '', 'removesuffix', '')
+        self.checkequal('', '', 'removesuffix', 'abcde')
+        self.checkequal('abcde', 'abcde', 'removesuffix', '')
+        self.checkequal('', 'abcde', 'removesuffix', 'abcde')
+
+        self.checkraises(TypeError, 'hello', 'removesuffix')
+        self.checkraises(TypeError, 'hello', 'removesuffix', 42)
+        self.checkraises(TypeError, 'hello', 'removesuffix', 42, 'h')
+        self.checkraises(TypeError, 'hello', 'removesuffix', 'h', 42)
+        self.checkraises(TypeError, 'hello', 'removesuffix', ("lo", "l"))
+
     def test_capitalize(self):
         self.checkequal(' hello ', ' hello ', 'capitalize')
         self.checkequal('Hello ', 'Hello ','capitalize')
diff --git a/Lib/test/test_doctest.py b/Lib/test/test_doctest.py
index 7142627..16196fe 100644
--- a/Lib/test/test_doctest.py
+++ b/Lib/test/test_doctest.py
@@ -665,7 +665,7 @@ plain ol' Python and is guaranteed to be available.
 
     >>> import builtins
     >>> tests = doctest.DocTestFinder().find(builtins)
-    >>> 810 < len(tests) < 830 # approximate number of objects with docstrings
+    >>> 816 < len(tests) < 836 # approximate number of objects with docstrings
     True
     >>> real_tests = [t for t in tests if len(t.examples) > 0]
     >>> len(real_tests) # objects that actually have doctests