Add pathclass.Extension class to enrich extension operations.

I often found myself writing code like if a.extension == 'png' and
trying to remember if I'm supposed to compare against 'png' or '.png',
and then it would trip up on files like A.PNG because I forgot to
lower() it.

So this class handles all that for you. You can == against it and it
will use os.path.normcase to give you OS-appropriate case sens,
and == works whether you include the dot or not. Then you can use
ext.with_dot or ext.no_dot to get reliably dotted strings.

voussoirkit/pathclass.py

@@ -15,6 +15,44 @@ class NotFile(PathclassException):
     pass
 
 
+class Extension:
+    def __init__(self, ext):
+        if isinstance(ext, Extension):
+            ext = ext.ext
+        ext = self.prep(ext)
+        self.ext = ext
+
+    @staticmethod
+    def prep(ext):
+        return os.path.normcase(ext).lstrip('.')
+
+    def __bool__(self):
+        return bool(self.ext)
+
+    def __eq__(self, other):
+        other = self.prep(other)
+        return self.ext == other
+
+    def __hash__(self):
+        return hash(self.ext)
+
+    def __repr__(self):
+        return f'Extension({repr(self.ext)})'
+
+    def __str__(self):
+        return self.ext
+
+    @property
+    def no_dot(self):
+        return self.ext
+
+    @property
+    def with_dot(self):
+        if self.ext == '':
+            return ''
+        return '.' + self.ext
+
+
 class Path:
     '''
     I started to use pathlib.Path, but it was too much of a pain.
@@ -65,10 +103,10 @@ class Path:
             raise NotDirectory(self)
 
     def add_extension(self, extension):
-        extension = extension.strip('.')
+        extension = Extension(extension)
         if extension == '':
             return self
-        return self.parent.with_child(self.basename + '.' + extension)
+        return self.parent.with_child(self.basename + extension.with_dot)
 
     @property
     def basename(self):
@@ -85,10 +123,7 @@ class Path:
 
     @property
     def dot_extension(self):
-        extension = self.extension
-        if extension:
-            return '.' + extension
-        return ''
+        return self.extension.with_dot
 
     @property
     def exists(self):
@@ -96,7 +131,7 @@ class Path:
 
     @property
     def extension(self):
-        return os.path.splitext(self.absolute_path)[1].lstrip('.')
+        return Extension(os.path.splitext(self.absolute_path)[1])
 
     @property
     def is_dir(self):
@@ -170,13 +205,13 @@ class Path:
         return relative_path
 
     def replace_extension(self, extension):
-        extension = extension.rsplit('.', 1)[-1]
+        extension = Extension(extension)
         base = os.path.splitext(self.basename)[0]
 
         if extension == '':
             return self.parent.with_child(base)
 
-        return self.parent.with_child(base + '.' + extension)
+        return self.parent.with_child(base + extension.with_dot)
 
     @property
     def size(self):