feat: leave previous answers intact when skipping question

copier/main.py

@@ -1,4 +1,5 @@
 """Main functions and classes, used to generate or update projects."""
+
 from __future__ import annotations
 
 import os
@@ -440,7 +441,8 @@ def _ask(self) -> None:
             # Skip a question when the skip condition is met.
             if not question.get_when():
                 # Omit its answer from the answers file.
-                result.hide(var_name)
+                if var_name not in result.last:
+                    result.hide(var_name)
                 # Skip immediately to the next question when it has no default
                 # value.
                 if question.default is MISSING:

docs/configuring.md

@@ -176,17 +176,18 @@ Supported keys:
     will be rendered with the combined answers as variables; it should render _nothing_
     if the value is valid, and an error message to show to the user otherwise.
 
--   **when**: Condition that, if `false`, skips the question.
+-   **when**: Condition that, if `false`, skips the question and doesn't record an answer.
 
-    If it is a boolean, it is used directly. Setting it to `false` is useful for
-    creating a computed value.
+    If it is a boolean, it is used directly.
 
-    If it is a string, it is converted to boolean using a parser similar to YAML, but
+    If it is a string, it is converted to a boolean using a parser similar to YAML, but
     only for boolean values. The string can be [templated][prompt-templating].
 
-    If a question is skipped, its answer is not recorded, but its default value is
+    If a question is skipped, its previous answer or default value is
     available in the render context.
 
+    Setting it to `false` together with default value is useful for creating a computed value.
+
     !!! example
 
         ```yaml title="copier.yaml"
@@ -199,6 +200,12 @@ Supported keys:
                 - GPLv3
                 - Public domain
 
+        year_of_first_publication:
+            type: str
+            # ask when running `copier copy`
+            # skip this question when running `copier update` in the future
+            when: "{{ year_of_creation is not defined }}"
+
         copyright_holder:
             type: str
             default: |-

tests/test_config.py

@@ -13,6 +13,7 @@
 from copier.errors import InvalidConfigFileError, MultipleConfigFilesError
 from copier.template import DEFAULT_EXCLUDE, Task, Template, load_template_config
 from copier.types import AnyByStrDict
+from copier.user_data import load_answersfile_data
 
 from .helpers import BRACKET_ENVOPS_JSON, SUFFIX_TMPL, build_file_tree
 
@@ -614,3 +615,51 @@ def test_secret_question_requires_default_value(
     build_file_tree({src / "copier.yml": config})
     with pytest.raises(ValueError, match="Secret question requires a default value"):
         copier.run_copy(str(src), dst)
+
+
+def test_falsy_when_doesnt_modify_previous_answer(
+    tmp_path_factory: pytest.TempPathFactory,
+) -> None:
+    src, dst = map(tmp_path_factory.mktemp, ("src", "dst"))
+    with local.cwd(src):
+        build_file_tree(
+            {
+                (src / "copier.yaml"): (
+                    """\
+                    initial_question:
+                        type: str
+                        default: "..."
+                        when: "{{initial_question is not defined}}"
+                    """
+                ),
+                (src / "{{ _copier_conf.answers_file }}.jinja"): (
+                    """\
+                    # Changes here will be overwritten by Copier
+                    {{ _copier_answers|to_nice_yaml }}
+                    """
+                ),
+            }
+        )
+        git_init()
+
+    copier.run_copy(
+        str(src),
+        dst,
+        defaults=True,
+        overwrite=True,
+    )
+
+    with local.cwd(dst):
+        git_init()
+
+    answers = load_answersfile_data(dst)
+    assert "initial_question" in answers
+
+    copier.run_update(
+        dst,
+        defaults=True,
+        overwrite=True,
+    )
+
+    answers = load_answersfile_data(dst)
+    assert "initial_question" in answers