fix(lib/cli): relative paths, empty slides, and tests (#223)

* fix(lib/cli): relative paths, empty slides, and tests

This fixes two issues:

1. Empty slides are now reported as error, to prevent indexing error;
2. Changing the folder path will now produce an absolute path to slides, which was not the case before and would lead to a "file does not exist error".

A few tests were also added to cover those

* fix(lib): fix from_file, remove useless field, and more

* chore(tests): remove print

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

manim_slides/config.py

@@ -1,4 +1,5 @@
 import hashlib
+import json
 import os
 import shutil
 import subprocess
@@ -7,7 +8,14 @@ from enum import Enum
 from pathlib import Path
 from typing import Any, Dict, List, Optional, Set, Tuple, Union
 
-from pydantic import BaseModel, FilePath, PositiveInt, field_validator, model_validator
+from pydantic import (
+    BaseModel,
+    Field,
+    FilePath,
+    PositiveInt,
+    field_validator,
+    model_validator,
+)
 from pydantic_extra_types.color import Color
 from PySide6.QtCore import Qt
 
@@ -71,6 +79,17 @@ class Config(BaseModel):  # type: ignore
     PLAY_PAUSE: Key = Key(ids=[Qt.Key_Space], name="PLAY / PAUSE")
     HIDE_MOUSE: Key = Key(ids=[Qt.Key_H], name="HIDE / SHOW MOUSE")
 
+    @classmethod
+    def from_file(cls, path: Path) -> "Config":
+        """Reads a configuration from a file."""
+        with open(path, "r") as f:
+            return cls.model_validate_json(f.read())  # type: ignore
+
+    def to_file(self, path: Path) -> None:
+        """Dumps the configuration to a file."""
+        with open(path, "w") as f:
+            f.write(self.model_dump_json(indent=2))
+
     @model_validator(mode="before")
     def ids_are_unique_across_keys(cls, values: Dict[str, Key]) -> Dict[str, Key]:
         ids: Set[int] = set()
@@ -104,7 +123,7 @@ class SlideConfig(BaseModel):  # type: ignore
     start_animation: int
     end_animation: int
     number: int
-    terminated: bool = False
+    terminated: bool = Field(False, exclude=True)
 
     @field_validator("start_animation", "end_animation")
     @classmethod
@@ -151,11 +170,31 @@ class SlideConfig(BaseModel):  # type: ignore
 
 
 class PresentationConfig(BaseModel):  # type: ignore
-    slides: List[SlideConfig]
+    slides: List[SlideConfig] = Field(min_length=1)
     files: List[FilePath]
     resolution: Tuple[PositiveInt, PositiveInt] = (1920, 1080)
     background_color: Color = "black"
 
+    @classmethod
+    def from_file(cls, path: Path) -> "PresentationConfig":
+        """Reads a presentation configuration from a file."""
+        with open(path, "r") as f:
+            obj = json.load(f)
+
+            if files := obj.get("files", None):
+                # First parent is ../slides
+                # so we take the parent of this parent
+                parent = Path(path).parents[1]
+                for i in range(len(files)):
+                    files[i] = parent / files[i]
+
+            return cls.model_validate(obj)  # type: ignore
+
+    def to_file(self, path: Path) -> None:
+        """Dumps the presentation configuration to a file."""
+        with open(path, "w") as f:
+            f.write(self.model_dump_json(indent=2))
+
     @model_validator(mode="after")
     def animation_indices_match_files(
         cls, config: "PresentationConfig"