fix: nargs + list as default value (PR #213, fixes #212)

CHANGES.rst

@@ -2,6 +2,19 @@
 Changelog
 ~~~~~~~~~
 
+Version 0.30.5 (2023-12-25)
+---------------------------
+
+Bugs fixed:
+
+- A combination of `nargs` with a list as default value would lead to the
+  values coming from CLI being wrapped in another list (issue #212).
+
+Enhancements:
+
+- Argspec guessing: if `nargs` is not specified but the default value
+  is a list, `nargs="*"` is assumed and passed to argparse.
+
 Version 0.30.4 (2023-11-04)
 ---------------------------
 

docs/source/cookbook.rst

@@ -10,10 +10,11 @@ Use `nargs` from argparse by amending the function signature with the
 .. code-block:: python
 
     @argh.arg("-p", "--patterns", nargs="*")
-    def cmd(patterns: list[str] | None = None) -> list:
+    def cmd(*, patterns: list[str] | None = None) -> list:
         distros = ("abc", "xyz")
-        return [d for d in distros if not patterns
-                                      or any(p in d for p in patterns)]
+        return [
+            d for d in distros if not patterns or any(p in d for p in patterns)
+        ]
 
 Resulting CLI::
 
@@ -34,3 +35,20 @@ Resulting CLI::
 
 Note that you need to specify both short and long names of the argument because
 `@arg` turns off the "smart" mechanism.
+
+In fact, you don't even need to use `nargs` if you specify a list as the
+default value (and provided that you're using the name mapping policy which
+will be eventually the default one):
+
+.. code-block:: python
+
+    def cmd(patterns: list[str] = ["default-pattern"]) -> list:
+        distros = ("abc", "xyz")
+        return [d for d in distros if any(p in d for p in patterns)]
+
+    if __name__ == "__main__":
+        parser = argh.ArghParser()
+        parser.set_default_command(
+            cmd, name_mapping_policy=argh.assembling.NameMappingPolicy.BY_NAME_IF_KWONLY
+        )
+        argh.dispatch(parser)

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "flit_core.buildapi"
 
 [project]
 name = "argh"
-version = "0.30.4"
+version = "0.30.5"
 description = "An unobtrusive argparse wrapper with natural syntax"
 readme = "README.rst"
 requires-python = ">=3.8"

src/argh/assembling.py

@@ -277,10 +277,16 @@ def guess_extra_parser_add_argument_spec_kwargs(
                 # (not applicable to positionals: _StoreAction doesn't accept `nargs`)
                 guessed["action"] = "store_false" if default_value else "store_true"
         elif other_add_parser_kwargs.get("type") is None:
-            # infer type from default value
-            # (make sure that action handler supports this keyword)
-            if other_add_parser_kwargs.get("action", "store") in TYPE_AWARE_ACTIONS:
-                guessed["type"] = type(default_value)
+            if isinstance(default_value, (list, tuple)):
+                if "nargs" not in other_add_parser_kwargs:
+                    # the argument has a default value, so it doesn't have to
+                    # be passed; "zero or more" is a reasonable guess
+                    guessed["nargs"] = ZERO_OR_MORE
+            else:
+                # infer type from default value
+                # (make sure that action handler supports this keyword)
+                if other_add_parser_kwargs.get("action", "store") in TYPE_AWARE_ACTIONS:
+                    guessed["type"] = type(default_value)
 
     # guess type from choices (first item)
     if other_add_parser_kwargs.get("choices") and "type" not in list(guessed) + list(

tests/test_regressions.py

@@ -3,7 +3,7 @@
 ~~~~~~~~~~~~~~~~
 """
 import sys
-from typing import TextIO
+from typing import List, Optional, TextIO
 
 import pytest
 
@@ -180,3 +180,65 @@ def func(foo_bar):
 
     parser = DebugArghParser()
     parser.set_default_command(func)
+
+
+def test_regression_issue212_orig_use_case():
+    """
+    Issue #212: a combination of nargs with list as default value would result
+    in a nested list instead of a flat list.
+
+    Variation: original use case (default value via decorator).
+    """
+
+    @argh.arg("paths", nargs="*", default=["one", "two"])
+    def func(paths: List[str]):
+        return f"{paths}"
+
+    parser = DebugArghParser()
+    parser.set_default_command(func)
+
+    assert run(parser, "").out == "['one', 'two']\n"
+    assert run(parser, "alpha").out == "['alpha']\n"
+    assert run(parser, "alpha beta gamma").out == "['alpha', 'beta', 'gamma']\n"
+
+
+def test_regression_issue212_funcsig_centric_positional():
+    """
+    Issue #212: a combination of nargs with list as default value would result
+    in a nested list instead of a flat list.
+
+    Variation: default value via function signature (positional).
+    """
+
+    @argh.arg("paths", nargs="*")
+    def func(paths: Optional[List[str]] = ["one", "two"]):
+        return f"{paths}"
+
+    parser = DebugArghParser()
+    parser.set_default_command(
+        func, name_mapping_policy=argh.assembling.NameMappingPolicy.BY_NAME_IF_KWONLY
+    )
+
+    assert run(parser, "").out == "['one', 'two']\n"
+    assert run(parser, "alpha").out == "['alpha']\n"
+    assert run(parser, "alpha beta gamma").out == "['alpha', 'beta', 'gamma']\n"
+
+
+def test_regression_issue212_funcsig_centric_named():
+    """
+    Issue #212: a combination of nargs with list as default value would result
+    in a nested list instead of a flat list.
+
+    Variation: default value via function signature (named).
+    """
+
+    @argh.arg("--paths", nargs="*")
+    def func(*, paths: Optional[List[str]] = ["one", "two"]):
+        return f"{paths}"
+
+    parser = DebugArghParser()
+    parser.set_default_command(func)
+
+    assert run(parser, "").out == "['one', 'two']\n"
+    assert run(parser, "--paths alpha").out == "['alpha']\n"
+    assert run(parser, "--paths alpha beta gamma").out == "['alpha', 'beta', 'gamma']\n"