code: def select_constraint(default, nightly=None, git_master=None):
      """Select dependency constraint based on TFX_DEPENDENCY_SELECTOR env var."""
      selector = os.environ.get("TFX_DEPENDENCY_SELECTOR")
      if selector == "UNCONSTRAINED":
          return ""
      elif selector == "NIGHTLY" and nightly is not None:
          return nightly
      elif selector == "GIT_MASTER" and git_master is not None:
          return git_master
      else:
          return default
  
imports: from distutils import log
  from distutils import spawn
  import os
  import platform
  import subprocess
  import sys
  from setuptools import Command
  from setuptools import find_packages
  from setuptools import setup
  from setuptools.command.build_py import build_py
  from setuptools.command.develop import develop
  from setuptools.command.egg_info import egg_info
  from setuptools.command.sdist import sdist
label_desc: Use image node_id 6 for calling a global function with example usage: select_constraint(default, nightly, git_master) and returns: str, nightly, git_master, default