docs: move prompt templates to files (#593)

* (docs): update docstrings, update custom-prompts.md
* (chore): enhance type hinting
* (fix): add missing `FileBasedPrompt` in module __init__.py

docs/custom-prompts.md

@@ -20,7 +20,9 @@ from pandasai.prompts import AbstractPrompt
 
 
 class MyCustomPrompt(AbstractPrompt):
-    template = """This is your custom text for your prompt with custom {my_custom_value}"""
+    @property
+    def template(self):
+        return """This is your custom text for your prompt with custom {my_custom_value}"""
 
 
 df = SmartDataframe("data.csv", {
@@ -31,6 +33,31 @@ df = SmartDataframe("data.csv", {
 })
 ```
 
+You can also use `FileBasedPrompt` in case you prefer to store prompt template in a file:
+
+_my_prompt_template.tmpl:_
+```
+This is your custom text for your prompt with custom {my_custom_value}
+```
+_python code:_
+
+```python
+from pandasai import SmartDataframe
+from pandasai.prompts import FileBasedPrompt
+
+
+class MyCustomFileBasedPrompt(FileBasedPrompt):
+    _path_to_template = "path/to/my_prompt_template.tmpl"
+
+
+df = SmartDataframe("data.csv", {
+    "custom_prompts": {
+        "generate_python_code": MyCustomFileBasedPrompt(
+            my_custom_value="my custom value")
+    }
+})
+```
+
 ## Using dynamic prompt values
 
 ### Variable interpolation

pandasai/prompts/__init__.py

@@ -1,9 +1,10 @@
-from .base import AbstractPrompt
+from .base import AbstractPrompt, FileBasedPrompt
 from .correct_error_prompt import CorrectErrorPrompt
 from .generate_python_code import GeneratePythonCodePrompt
 
 __all__ = [
     "AbstractPrompt",
     "CorrectErrorPrompt",
     "GeneratePythonCodePrompt",
+    "FileBasedPrompt",
 ]

pandasai/prompts/base.py

@@ -6,7 +6,10 @@
 
 
 class AbstractPrompt(ABC):
-    """Base class to implement a new Prompt"""
+    """Base class to implement a new Prompt.
+
+    Inheritors have to override `template` property.
+    """
 
     _args = {}
 
@@ -47,7 +50,7 @@ def _generate_dataframes(self, dfs):
 
     @property
     @abstractmethod
-    def template(self):
+    def template(self) -> str:
         ...
 
     def set_var(self, var, value):
@@ -63,6 +66,11 @@ def __str__(self):
 
 
 class FileBasedPrompt(AbstractPrompt):
+    """Base class for prompts supposed to read template content from a file.
+
+    `_path_to_template` attribute has to be specified.
+    """
+
     _path_to_template: str
 
     def __init__(self, **kwargs):
@@ -72,7 +80,7 @@ def __init__(self, **kwargs):
         super().__init__(**kwargs)
 
     @property
-    def template(self):
+    def template(self) -> str:
         if not os.path.exists(self._path_to_template):
             raise FileNotFoundError(
                 f"Unable to find a file with template at '{self._path_to_template}' "