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: import os
  import platform
  import shutil
  import subprocess
  import sys
  import setuptools
  from setuptools import find_packages
  from setuptools import setup
  from setuptools.command.install import install
  from setuptools.dist import Distribution
  from distutils.command import build
label_desc: Use image node_id 4 for calling a global function with example usage: select_constraint(default, nightly, git_master) and returns: str, nightly, git_master, default