cli: create superclass for output formatters

This eases the isinstance check for Blocks, Tables and whatnot by
creating an abstract superclass which represents those, defining
to_string in its interface for consistency.

Signed-off-by: Renan Rodrigo <[email protected]>

uaclient/cli/formatter.py

@@ -1,3 +1,4 @@
+import abc
 import os
 import re
 import sys
@@ -97,7 +98,16 @@ def wrap_text(text: str, max_width: int) -> List[str]:
     return wrapped_lines
 
 
-class Table:
+class ProOutputFormatter(abc.ABC):
+    @abc.abstractmethod
+    def to_string(self, line_length: Optional[int] = None) -> str:
+        pass
+
+    def __str__(self):
+        return self.to_string()
+
+
+class Table(ProOutputFormatter):
     SEPARATOR = " " * 2
 
     def __init__(
@@ -148,9 +158,6 @@ def _get_column_sizes(self) -> List[int]:
 
         return column_sizes
 
-    def __str__(self) -> str:
-        return self.to_string()
-
     def to_string(self, line_length: Optional[int] = None) -> str:
         if line_length is None:
             line_length = _get_default_length()
@@ -204,7 +211,7 @@ def _fill_row(self, row: List[str]) -> str:
         return output
 
 
-class Block:
+class Block(ProOutputFormatter):
     INDENT_SIZE = 4
     INDENT_CHAR = " "
 
@@ -216,9 +223,6 @@ def __init__(
         self.title = title
         self.content = content if content is not None else []
 
-    def __str__(self) -> str:
-        return self.to_string()
-
     def to_string(self, line_length: Optional[int] = None) -> str:
         if line_length is None:
             line_length = _get_default_length()
@@ -237,7 +241,7 @@ def to_string(self, line_length: Optional[int] = None) -> str:
             )
 
         for item in self.content:
-            if isinstance(item, (Block, Table)):
+            if isinstance(item, ProOutputFormatter):
                 item_str = item.to_string(line_length=line_length)
             else:
                 item_str = "\n".join(wrap_text(str(item), line_length)) + "\n"