Added example for using CompletionItems as elements in an argparse choices list. (#1644)

Author: kmvanbrunt

cmd2/completion.py

@@ -49,14 +49,16 @@ class CompletionItem:
     # control sequences (like ^J or ^I) in the completion menu.
     _CONTROL_WHITESPACE_RE = re.compile(r"\r\n|[\n\r\t\f\v]")
 
-    # The core object this completion represents (e.g., str, int, Path).
-    # This serves as the default source for the completion string and is used
-    # to support object-based validation when used in argparse choices.
+    # The source input for the completion. This is used to initialize the 'text'
+    # field (defaults to str(value)). The original object is also preserved to
+    # support object-based validation when this CompletionItem is used as an
+    # argparse choice.
     value: Any = field(kw_only=False)
 
-    # The actual completion string. If not provided, defaults to str(value).
-    # This can be used to provide a human-friendly alias for complex objects in
-    # an argparse choices list (requires a matching 'type' converter for validation).
+    # The string matched against user input and inserted into the command line.
+    # Defaults to str(value). This should only be set manually if this
+    # CompletionItem is used as an argparse choice and you want the choice
+    # string to differ from str(value).
     text: str = _UNSET_STR
 
     # Optional string for displaying the completion differently in the completion menu.

examples/README.md

@@ -34,6 +34,8 @@ each:
     - Example that demonstrates the `CommandSet` features for modularizing commands and demonstrates
       all main capabilities including basic CommandSets, dynamic loading an unloading, using
       subcommands, etc.
+- [completion_item_choices.py](https://github.com/python-cmd2/cmd2/blob/main/examples/completion_item_choices.py)
+    - Demonstrates using CompletionItem instances as elements in an argparse choices list.
 - [custom_parser.py](https://github.com/python-cmd2/cmd2/blob/main/examples/custom_parser.py)
     - Demonstrates how to create your own custom `Cmd2ArgumentParser`
 - [custom_types.py](https://github.com/python-cmd2/cmd2/blob/main/examples/custom_types.py)

examples/completion_item_choices.py

@@ -0,0 +1,145 @@
+#!/usr/bin/env python
+"""
+Demonstrates using CompletionItem instances as elements in an argparse choices list.
+
+Technical Note:
+  Using 'choices' is best for fixed datasets that do not change during the
+  application's lifecycle. For dynamic data (e.g., results from a database or
+  file system), use a 'choices_provider' instead.
+
+Key strengths of this approach:
+  1. Command handlers receive fully-typed domain objects directly in the
+     argparse.Namespace, eliminating manual lookups from string keys.
+  2. Choices carry tab-completion UI enhancements (display_meta, table_data)
+     that are not supported by standard argparse string choices.
+  3. Provides a single source of truth for completion UI, input validation,
+     and object mapping.
+
+This demo showcases two distinct approaches:
+  1. Simple: Using CompletionItems with basic types (ints) to add UI metadata
+     (display_meta) while letting argparse handle standard type conversion.
+  2. Advanced: Using a custom 'text' alias and a type converter to map a friendly
+     string (e.g., 'alice') directly to a complex object (Account).
+"""
+
+import argparse
+import sys
+from typing import (
+    ClassVar,
+    cast,
+)
+
+from cmd2 import (
+    Cmd,
+    Cmd2ArgumentParser,
+    CompletionItem,
+    with_argparser,
+)
+
+# -----------------------------------------------------------------------------
+# Simple Example: Basic types with UI metadata
+# -----------------------------------------------------------------------------
+# Integers with metadata. No 'text' override or custom type converter needed.
+# argparse will handle 'type=int' and validate it against the CompletionItem.value.
+id_choices = [
+    CompletionItem(101, display_meta="Alice's Account"),
+    CompletionItem(202, display_meta="Bob's Account"),
+]
+
+
+# -----------------------------------------------------------------------------
+# Advanced Example: Mapping friendly aliases to objects
+# -----------------------------------------------------------------------------
+class Account:
+    """A complex object that we want to select by a friendly name."""
+
+    def __init__(self, account_id: int, owner: str):
+        self.account_id = account_id
+        self.owner = owner
+
+    def __eq__(self, other: object) -> bool:
+        if isinstance(other, Account):
+            return self.account_id == other.account_id
+        return False
+
+    def __hash__(self) -> int:
+        return hash(self.account_id)
+
+    def __repr__(self) -> str:
+        return f"Account(id={self.account_id}, owner='{self.owner}')"
+
+
+# Map friendly 'text' aliases to the actual object 'value'.
+# The user types 'alice' or 'bob' (tab-completion), but the parsed value will be the Account object.
+accounts = [
+    Account(101, "Alice"),
+    Account(202, "Bob"),
+]
+account_choices = [
+    CompletionItem(
+        acc,
+        text=acc.owner.lower(),
+        display_meta=f"ID: {acc.account_id}",
+    )
+    for acc in accounts
+]
+
+
+def account_lookup(name: str) -> Account:
+    """Type converter that looks up an Account by its friendly name."""
+    for item in account_choices:
+        if item.text == name:
+            return cast(Account, item.value)
+    raise argparse.ArgumentTypeError(f"invalid account: {name}")
+
+
+# -----------------------------------------------------------------------------
+# Demo Application
+# -----------------------------------------------------------------------------
+class ChoicesDemo(Cmd):
+    """Demo cmd2 application."""
+
+    DEFAULT_CATEGORY: ClassVar[str] = "Demo Commands"
+
+    def __init__(self) -> None:
+        super().__init__()
+        self.intro = (
+            "Welcome to the CompletionItem Choices Demo!\n"
+            "Try 'simple' followed by [TAB] to see basic metadata.\n"
+            "Try 'advanced' followed by [TAB] to see custom string mapping."
+        )
+
+    # Simple Command: argparse handles the int conversion, CompletionItem handles the UI
+    simple_parser = Cmd2ArgumentParser()
+    simple_parser.add_argument(
+        "account_id",
+        type=int,
+        choices=id_choices,
+        help="Select an account ID (tab-complete to see metadata)",
+    )
+
+    @with_argparser(simple_parser)
+    def do_simple(self, args: argparse.Namespace) -> None:
+        """Show an account ID selection (Simple Case)."""
+        # argparse converted the input to an int, and validated it against the CompletionItem.value
+        self.poutput(f"Selected Account ID: {args.account_id} (Type: {type(args.account_id).__name__})")
+
+    # Advanced Command: Custom lookup and custom 'text' mapping
+    advanced_parser = Cmd2ArgumentParser()
+    advanced_parser.add_argument(
+        "account",
+        type=account_lookup,
+        choices=account_choices,
+        help="Select an account by owner name (tab-complete to see friendly names)",
+    )
+
+    @with_argparser(advanced_parser)
+    def do_advanced(self, args: argparse.Namespace) -> None:
+        """Show a custom string selection (Advanced Case)."""
+        # args.account is the full Account object
+        self.poutput(f"Selected Account: {args.account!r} (Type: {type(args.account).__name__})")
+
+
+if __name__ == "__main__":
+    app = ChoicesDemo()
+    sys.exit(app.cmdloop())