apply changes as suggested @psychedelicious in PR comments.

- filename -> file_path
- pre and post prompt changed to optional
- clearer pre and post prompt descriptions
- handle pre and post prompt passed as None
- max_prompts defaults to 1 isted of 0 to avoid accidentally processing large prompt files with it set to 0 when adding a new node.

invokeai/app/invocations/prompt.py

@@ -1,7 +1,8 @@
-from typing import Literal
+from os.path import exists
+from typing import Literal, Optional
 
 import numpy as np
-from pydantic.fields import Field
+from pydantic import Field, validator
 
 from .baseinvocation import BaseInvocation, BaseInvocationOutput, InvocationContext
 from dynamicprompts.generators import RandomPromptGenerator, CombinatorialPromptGenerator
@@ -64,27 +65,33 @@ class PromptsFromFileInvocation(BaseInvocation):
     type: Literal['prompt_from_file'] = 'prompt_from_file'
 
     # Inputs
-    filename: str = Field(default=None, description="Filename of prompt text file")
-    pre_prompt: str = Field(default=None, description="Add to start of prompt")
-    post_prompt: str = Field(default=None, description="Add to end of prompt")
-    start_line: int = Field(default=1, ge=1, description="Line in the file start start from")
-    max_prompts: int = Field(default=0, ge=0, description="Max lines to read from file (0=all)")
+    file_path: str = Field(description="Path to prompt text file")
+    pre_prompt: Optional[str] = Field(description="String to prepend to each prompt")
+    post_prompt: Optional[str] = Field(description="String to append to each prompt")
+    start_line: int = Field(default=1, ge=1, description="Line in the file to start start from")
+    max_prompts: int = Field(default=1, ge=0, description="Max lines to read from file (0=all)")
     #fmt: on
 
-    def promptsFromFile(self, filename: str, pre_prompt: str, post_prompt: str, start_line: int, max_prompts:int):
+    @validator("file_path")
+    def file_path_exists(cls, v):
+        if not exists(v):
+            raise ValueError("file path not found")
+        return v
+
+    def promptsFromFile(self, file_path: str, pre_prompt: str, post_prompt: str, start_line: int, max_prompts: int):
         prompts = []
         start_line -= 1
         end_line = start_line + max_prompts
         if max_prompts <= 0:
             end_line = np.iinfo(np.int32).max
-        with open(filename) as f:
+        with open(file_path) as f:
             for i, line in enumerate(f):
                 if i >= start_line and i < end_line:
-                    prompts.append(pre_prompt + line.strip() + post_prompt)
+                    prompts.append((pre_prompt or '') + line.strip() + (post_prompt or ''))
                 if i >= end_line:
                     break
         return prompts
 
     def invoke(self, context: InvocationContext) -> PromptCollectionOutput:
-        prompts = self.promptsFromFile(self.filename, self.pre_prompt, self.post_prompt, self.start_line, self.max_prompts)
+        prompts = self.promptsFromFile(self.file_path, self.pre_prompt, self.post_prompt, self.start_line, self.max_prompts)
         return PromptCollectionOutput(prompt_collection=prompts, count=len(prompts))