feat: add ability to show unified diffs for changed files

copier/cli.py

@@ -92,6 +92,7 @@ class CopierApp(cli.Application):
         skip: Set [skip_if_exists][] option.
         prereleases: Set [use_prereleases][] option.
         quiet: Set [quiet][] option.
+        diff: Set [diff][] option.
     """
 
     DESCRIPTION = "Create a new project from a template."
@@ -175,6 +176,10 @@ class CopierApp(cli.Application):
         ["-g", "--prereleases"],
         help="Use prereleases to compare template VCS tags.",
     )
+    diff: cli.Flag = cli.Flag(
+        ["-D", "--diff"],
+        help="Show diff when updating files",
+    )
 
     @cli.switch(
         ["-d", "--data"],
@@ -216,6 +221,7 @@ def _worker(self, src_path: OptStr = None, dst_path: str = ".", **kwargs) -> Wor
             src_path=src_path,
             vcs_ref=self.vcs_ref,
             use_prereleases=self.prereleases,
+            diff=self.diff,
             **kwargs,
         )
 

copier/main.py

@@ -12,6 +12,7 @@
 from typing import Callable, Iterable, List, Mapping, Optional, Sequence
 from unicodedata import normalize
 
+import colorama
 import pathspec
 from jinja2.loaders import FileSystemLoader
 from jinja2.sandbox import SandboxedEnvironment
@@ -26,7 +27,7 @@
 from .errors import CopierAnswersInterrupt, ExtensionNotFoundError, UserMessageError
 from .subproject import Subproject
 from .template import Template
-from .tools import Style, TemporaryDirectory, printf
+from .tools import Style, TemporaryDirectory, color_print, is_binary, print_diff, printf
 from .types import (
     AnyByStrDict,
     JSONSerializable,
@@ -128,6 +129,12 @@ class Worker:
             When `True`, disable all output.
 
             See [quiet][].
+
+        diff:
+            When `True` show a unified diff for changed files.
+
+            See [diff][].
+
     """
 
     src_path: Optional[str] = None
@@ -144,6 +151,7 @@ class Worker:
     overwrite: bool = False
     pretend: bool = False
     quiet: bool = False
+    diff: bool = False
 
     def _answers_to_remember(self) -> Mapping:
         """Get only answers that will be remembered in the copier answers file."""
@@ -216,7 +224,12 @@ def _path_matcher(self, patterns: Iterable[str]) -> Callable[[Path], bool]:
         spec = pathspec.PathSpec.from_lines("gitwildmatch", normalized_patterns)
         return spec.match_file
 
-    def _solve_render_conflict(self, dst_relpath: Path):
+    def _solve_render_conflict(
+        self,
+        dst_relpath: Path,
+        original_content: Optional[bytes] = None,
+        new_content: Optional[bytes] = None,
+    ):
         """Properly solve render conflicts.
 
         It can ask the user if running in interactive mode.
@@ -247,6 +260,11 @@ def _solve_render_conflict(self, dst_relpath: Path):
                 file_=sys.stderr,
             )
             return True
+        if self.diff and original_content and new_content:
+            if is_binary(original_content) or is_binary(new_content):
+                color_print(colorama.Fore.YELLOW, "    diff not availalbe for binaries")
+            else:
+                print_diff(dst_relpath, original_content, new_content)
         return bool(ask(f" Overwrite {dst_relpath}?", default=True))
 
     def _render_allowed(
@@ -310,7 +328,9 @@ def _render_allowed(
                     file_=sys.stderr,
                 )
                 return True
-            return self._solve_render_conflict(dst_relpath)
+            return self._solve_render_conflict(
+                dst_relpath, previous_content, expected_contents
+            )
 
     @cached_property
     def answers(self) -> AnswersMap:

copier/tools.py

@@ -8,6 +8,7 @@
 import tempfile
 import warnings
 from contextlib import suppress
+from difflib import unified_diff
 from pathlib import Path
 from types import TracebackType
 from typing import Any, Callable, Optional, TextIO, Tuple, Union
@@ -16,7 +17,7 @@
 from packaging.version import Version
 from pydantic import StrictBool
 
-from .types import IntSeq
+from .types import IntSeq, StrSeq
 
 try:
     from importlib.metadata import version
@@ -172,3 +173,56 @@ def cleanup(self):
     @staticmethod
     def _robust_cleanup(name):
         shutil.rmtree(name, ignore_errors=False, onerror=handle_remove_readonly)
+
+
+def get_lines(content: bytes) -> StrSeq:
+    # Assume UTF-8 encoding for text files. Since Jinja2 is already doing this
+    # for templates, there  probably isn't much harm in also doing it for all
+    # text files. The worst case should be a failure to show the diffs. An
+    # alternative is to add chardet support.
+    return content.decode("utf-8").split(os.linesep)
+
+
+def is_binary(content: bytes) -> bool:
+    """Return a boolean indicating if the content is a binary file."""
+    return b"\x00" in content or b"\xff" in content
+
+
+def color_print(color: int, msg: str):
+    print(f"{color}{msg}{colorama.Fore.RESET}")
+
+
+def print_diff(dst_relpath: Path, original_content: bytes, new_content: bytes):
+    try:
+        original_lines = get_lines(original_content)
+    except Exception:
+        color_print(
+            colorama.Fore.YELLOW, "diff unavailable: unable to determine encoding"
+        )
+        return
+    try:
+        new_lines = get_lines(new_content)
+    except Exception:
+        color_print(
+            colorama.Fore.YELLOW, "diff unavailable: unable to determine encoding"
+        )
+        return
+
+    indent = " " * 6
+    for line in unified_diff(
+        original_lines,
+        new_lines,
+        f"old/{dst_relpath}",
+        f"new/{dst_relpath}",
+        lineterm="",
+    ):
+        if line.startswith("---") or line.startswith("+++"):
+            color_print(colorama.Fore.YELLOW, f"{indent}{line}")
+        elif line.startswith("@@"):
+            color_print(colorama.Fore.MAGENTA, f"{indent}{line}")
+        elif line.startswith("-"):
+            color_print(colorama.Fore.RED, f"{indent}{line}")
+        elif line.startswith("+"):
+            color_print(colorama.Fore.GREEN, f"{indent}{line}")
+        else:
+            print(f"{indent}{line}")

tests/test_templated_prompt.py

@@ -217,7 +217,7 @@ def test_templated_prompt_builtins(tmp_path_factory):
     )
     Worker(str(src), dst, defaults=True, overwrite=True).run_copy()
     that_now = datetime.fromisoformat((dst / "now").read_text())
-    assert that_now <= datetime.now()
+    assert that_now <= datetime.utcnow()
     assert len((dst / "make_secret").read_text()) == 128