Add a constant with all expr-variables in order.

vsutil/types.py

@@ -1,7 +1,7 @@
 """
 Enums and related functions.
 """
-__all__ = ['Dither', 'Range']
+__all__ = ['Dither', 'Range', 'EXPR_VARS']
 
 # this file only depends on the stdlib and should stay that way
 from enum import Enum
@@ -40,6 +40,16 @@ class Range(_NoSubmoduleRepr, int, Enum):
     """Full (PC) dynamic range, 0-255 in 8 bits."""
 
 
+EXPR_VARS: str = 'xyzabcdefghijklmnopqrstuvw'
+"""
+This constant contains a list of all variables that can appear inside an expr-string ordered
+by assignment. So the first clip will have the name EXPR_VAR_NAMES[0], the second one will
+have the name EXPR_VAR_NAMES[1], and so on.
+
+This can be used to automatically generate Expr-strings.
+"""    
+
+
 def _readable_enums(enum: Type[Enum]) -> str:
     """
     Returns a list of all possible values in `enum`.