class_docstr: A Cell serves a Direct Acyclic Graph (DAG) which holds blocks as nodes and edges that
  connects operation paths between the nodes.
class_docstr_tok: ["A","Cell","serves","a","Direct","Acyclic","Graph","(","DAG",")","which","holds","blocks","as","nodes","and","edges","that","connects","operation","paths","between","the","nodes","."]
docstr: Index of the input node.
docstr_tok: ["Index","of","the","input","node","."]
code: def input_idx(self) -> int:
      """Index of the input node."""
  
      for node in self.nodes:
          if self.nodes[node]["block"].type == "input":
              return node
  
      return -1
  
file_path: opytimizer/opytimizer/core/cell.py
imports: import copy
  from typing import Generator, Tuple
  import networkx
  from networkx import DiGraph
  from opytimizer.core import InnerBlock, InputBlock, OutputBlock
  from opytimizer.core.block import Block
label_desc: Use image node_id 3 for calling the Cell obj's underlying member method code with example usage: obj.input_idx() and returns: unknown, node