flag for file existence checks on add_static_file (#4411)

- Added flag `strict` to `add_static_file` params that determines
whether to check if file exists or not.
- default value should be FALSE because:
1) python stdlibs like
[pathlib](https://docs.python.org/3/library/pathlib.html) are default
false, because paths are not checked for existence until accessed
2) existing code is very unlikely to use `add_static_file` raising an
error to determine whether a file exists or not
3) raising error on file existence is undocumented behavior, making it
even less likely for existing code to rely on
- changed error raised from general ValueError to more appropriate
FileNotFoundError as used by stdlib

---------

Co-authored-by: Admin <foobar@morboa.local>
Co-authored-by: Falko Schindler <falko@zauberzeug.com>

nicegui/app/app.py

@@ -175,6 +175,7 @@ class App(FastAPI):
                         local_file: Union[str, Path],
                         url_path: Optional[str] = None,
                         single_use: bool = False,
+                        strict: bool = True,
                         max_cache_age: int = 3600) -> str:
         """Add a single static file.
 
@@ -184,9 +185,13 @@ class App(FastAPI):
         To make a whole folder of files accessible, use `add_static_files()` instead.
         For media files which should be streamed, you can use `add_media_files()` or `add_media_file()` instead.
 
+        Deprecation warning:
+        Non-existing files will raise a ``FileNotFoundError`` rather than a ``ValueError`` in version 3.0 if ``strict`` is ``True``.
+
         :param local_file: local file to serve as static content
         :param url_path: string that starts with a slash "/" and identifies the path at which the file should be served (default: None -> auto-generated URL path)
         :param single_use: whether to remove the route after the file has been downloaded once (default: False)
+        :param strict: whether to raise a ``ValueError`` if the file does not exist (default: False, *added in version 2.12.0*)
         :param max_cache_age: value for max-age set in Cache-Control header (*added in version 2.8.0*)
         :return: encoded URL which can be used to access the file
         """
@@ -194,8 +199,8 @@ class App(FastAPI):
             raise ValueError('''Value of max_cache_age must be a positive integer or 0.''')
 
         file = Path(local_file).resolve()
-        if not file.is_file():
-            raise ValueError(f'File not found: {file}')
+        if strict and not file.is_file():
+            raise ValueError(f'File not found: {file}')  # DEPRECATED: will raise a ``FileNotFoundError`` in version 3.0
         path = f'/_nicegui/auto/static/{helpers.hash_file_path(file)}/{file.name}' if url_path is None else url_path
 
         @self.get(path)
@@ -231,7 +236,7 @@ class App(FastAPI):
                        local_file: Union[str, Path],
                        url_path: Optional[str] = None,
                        single_use: bool = False,
-                       ) -> str:
+                       strict: bool = True) -> str:
         """Add a single media file.
 
         Allows a local file to be streamed.
@@ -240,14 +245,18 @@ class App(FastAPI):
         To make a whole folder of media files accessible via streaming, use `add_media_files()` instead.
         For small static files, you can use `add_static_files()` or `add_static_file()` instead.
 
+        Deprecation warning:
+        Non-existing files will raise a ``FileNotFoundError`` rather than a ``ValueError`` in version 3.0 if ``strict`` is ``True``.
+
         :param local_file: local file to serve as media content
         :param url_path: string that starts with a slash "/" and identifies the path at which the file should be served (default: None -> auto-generated URL path)
         :param single_use: whether to remove the route after the media file has been downloaded once (default: False)
+        :param strict: whether to raise a ``ValueError`` if the file does not exist (default: False, *added in version 2.12.0*)
         :return: encoded URL which can be used to access the file
         """
         file = Path(local_file).resolve()
-        if not file.is_file():
-            raise ValueError(f'File not found: {local_file}')
+        if strict and not file.is_file():
+            raise ValueError(f'File not found: {file}')  # DEPRECATED: will raise a ``FileNotFoundError`` in version 3.0
         path = f'/_nicegui/auto/media/{helpers.hash_file_path(file)}/{file.name}' if url_path is None else url_path
 
         @self.get(path)