add strict option to strip prefix/suffix, which can be used to ensure…

… the stripped substring was present

tests/test_util.py

@@ -36,6 +36,16 @@ def test_strip_suffix(in_, suf, out):
     assert strip_suffix(in_, suf) == out
 
 
+def test_strip_prefix_strict():
+    with pytest.raises(WimpyError, match="string doesn't start with prefix"):
+        strip_prefix("www.yahoo.com", "ww.", strict=True)
+
+
+def test_strip_suffix_strict():
+    with pytest.raises(WimpyError, match="string doesn't end with suffix"):
+        strip_suffix("www.yahoo.com", ".org", strict=True)
+
+
 def test_working_directory(tmpdir):
     before = os.getcwd()
     with working_directory(tmpdir):

wimpy/util.py

@@ -48,17 +48,23 @@ def working_directory(path):
         os.chdir(prev_dir)
 
 
-def strip_prefix(s, prefix):
-    """Removes the prefix, if it's there, otherwise returns input string unchanged"""
+def strip_prefix(s, prefix, strict=False):
+    """Removes the prefix, if it's there, otherwise returns input string unchanged.
+    If strict is True, also ensures the prefix was present"""
     if s.startswith(prefix):
         return s[len(prefix) :]
+    elif strict:
+        raise WimpyError("string doesn't start with prefix")
     return s
 
 
-def strip_suffix(s, suffix):
-    """Removes the suffix, if it's there, otherwise returns input string unchanged"""
+def strip_suffix(s, suffix, strict=False):
+    """Removes the suffix, if it's there, otherwise returns input string unchanged.
+    If strict is True, also ensures the suffix was present"""
     if s.endswith(suffix):
         return s[: len(s) - len(suffix)]
+    elif strict:
+        raise WimpyError("string doesn't end with suffix")
     return s