Warn major version difference

Targeting #348, from now on, if we cannot check the copier version or the installed version's major part is bigger than the template's minimal major part, there'll be proper warnings.

copier/errors.py

@@ -8,19 +8,24 @@
 from .types import PathSeq
 
 
-class UserMessageError(Exception):
+# Errors
+class CopierError(Exception):
+    """Base class for all other Copier errors."""
+
+
+class UserMessageError(CopierError):
     """Exit the program giving a message to the user."""
 
 
-class UnsupportedVersionError(UserMessageError):
+class UnsupportedVersionError(UserMessageError, CopierError):
     """Copier version does not support template version."""
 
 
-class ConfigFileError(ValueError):
+class ConfigFileError(ValueError, CopierError):
     """Parent class defining problems with the config file."""
 
 
-class InvalidConfigFileError(ConfigFileError):
+class InvalidConfigFileError(ConfigFileError, CopierError):
     """Indicates that the config file is wrong."""
 
     def __init__(self, conf_path: Path, quiet: bool):
@@ -29,7 +34,7 @@ def __init__(self, conf_path: Path, quiet: bool):
         super().__init__(msg)
 
 
-class MultipleConfigFilesError(ConfigFileError):
+class MultipleConfigFilesError(ConfigFileError, CopierError):
     """Both copier.yml and copier.yaml found, and that's an error."""
 
     def __init__(self, conf_paths: "PathSeq", quiet: bool):
@@ -38,19 +43,32 @@ def __init__(self, conf_paths: "PathSeq", quiet: bool):
         super().__init__(msg)
 
 
-class InvalidTypeError(TypeError):
+class InvalidTypeError(TypeError, CopierError):
     """The question type is not among the supported ones."""
 
 
-class PathNotAbsoluteError(_PathValueError):
+class PathNotAbsoluteError(_PathValueError, CopierError):
     """The path is not absolute, but it should be."""
 
     code = "path.not_absolute"
     msg_template = '"{path}" is not an absolute path'
 
 
-class PathNotRelativeError(_PathValueError):
+class PathNotRelativeError(_PathValueError, CopierError):
     """The path is not relative, but it should be."""
 
     code = "path.not_relative"
     msg_template = '"{path}" is not a relative path'
+
+
+# Warnings
+class CopierWarning(Warning):
+    """Base class for all other Copier warnings."""
+
+
+class UnknownCopierVersionWarning(UserWarning, CopierWarning):
+    """Cannot determine installed Copier version."""
+
+
+class OldTemplateWarning(UserWarning, CopierWarning):
+    """Template was designed for an older Copier version."""

copier/template.py

@@ -2,14 +2,18 @@
 from contextlib import suppress
 from pathlib import Path
 from typing import List, Mapping, Optional, Sequence, Set, Tuple
+from warnings import warn
 
-from packaging import version
-from packaging.version import parse
+from packaging.version import Version, parse
 from plumbum.cmd import git
 from plumbum.machines import local
 from pydantic.dataclasses import dataclass
 
-from .errors import UnsupportedVersionError
+from .errors import (
+    OldTemplateWarning,
+    UnknownCopierVersionWarning,
+    UnsupportedVersionError,
+)
 from .types import AnyByStrDict, OptStr, StrSeq, VCSTypes
 from .user_data import load_config_data
 from .vcs import checkout_latest_tag, clone, get_repo
@@ -53,22 +57,38 @@ def filter_config(data: AnyByStrDict) -> Tuple[AnyByStrDict, AnyByStrDict]:
     return conf_data, questions_data
 
 
-def verify_minimum_version(version_str: str) -> None:
-    """Raise an error if the current Copier version is less than the given version."""
+def verify_copier_version(version_str: str) -> None:
+    """Raise an error if the current Copier version is less than the given version.
+
+    Args:
+        version_str:
+            Minimal copier version for the template.
+    """
     # Importing __version__ at the top of the module creates a circular import
     # ("cannot import name '__version__' from partially initialized module 'copier'"),
     # so instead we do a lazy import here
     from . import __version__
 
     # Disable check when running copier as editable installation
     if __version__ == "0.0.0":
+        warn(
+            "Cannot check Copier version constraint.",
+            UnknownCopierVersionWarning,
+        )
         return
-
-    if version.parse(__version__) < version.parse(version_str):
+    parsed_installed, parsed_min = map(Version, (__version__, version_str))
+    if parsed_installed < parsed_min:
         raise UnsupportedVersionError(
             f"This template requires Copier version >= {version_str}, "
             f"while your version of Copier is {__version__}."
         )
+    if parsed_installed.major > parsed_min.major:
+        warn(
+            f"This template was designed for Copier {version_str}, "
+            f"but your version of Copier is {__version__}. "
+            f"You could find some incompatibilities.",
+            OldTemplateWarning,
+        )
 
 
 @dataclass
@@ -120,7 +140,7 @@ def _raw_config(self) -> AnyByStrDict:
         """
         result = load_config_data(self.local_abspath)
         with suppress(KeyError):
-            verify_minimum_version(result["_min_copier_version"])
+            verify_copier_version(result["_min_copier_version"])
         return result
 
     @cached_property

tests/test_minimum_version.py

@@ -1,9 +1,15 @@
+import warnings
+
 import pytest
 from plumbum import local
 from plumbum.cmd import git
 
 import copier
-from copier.errors import UnsupportedVersionError
+from copier.errors import (
+    OldTemplateWarning,
+    UnknownCopierVersionWarning,
+    UnsupportedVersionError,
+)
 
 from .helpers import build_file_tree
 
@@ -68,4 +74,15 @@ def test_minimum_version_update(template_path, tmp_path, monkeypatch):
 def test_version_0_0_0_ignored(template_path, tmp_path, monkeypatch):
     monkeypatch.setattr("copier.__version__", "0.0.0")
     # assert no error
-    copier.copy(template_path, tmp_path)
+    with warnings.catch_warnings():
+        warnings.simplefilter("error")
+        with pytest.raises(UnknownCopierVersionWarning):
+            copier.run_copy(template_path, tmp_path)
+
+
+def test_version_bigger_major_warning(template_path, tmp_path, monkeypatch):
+    monkeypatch.setattr("copier.__version__", "11.0.0a0")
+    with warnings.catch_warnings():
+        warnings.simplefilter("error")
+        with pytest.raises(OldTemplateWarning):
+            copier.run_copy(template_path, tmp_path)