class_docstr: A progress bar based on tqdm.
  
  This callback is one of the :func:`DEFAULT_CALLBACKS()`.
class_docstr_tok: ["A","progress","bar","based","on","tqdm",".","This","callback","is","one","of","the",":","func",":","`","DEFAULT_CALLBACKS","(",")","`","."]
code: def _before_run(self, _):
      # update progress bar when local step changed (one step is finished)
      if self.local_step != self._last_updated:
          self._last_updated = self.local_step
          return self._fetches
      else:
          return None
  
imports: import tqdm
  from six.moves import zip
  from ..compat import tfv1
  from ..tfutils.common import get_global_step_var, get_op_tensor_name
  from ..utils import logger
  from ..utils.naming import GLOBAL_STEP_INCR_OP_NAME
  from ..utils.utils import get_tqdm_kwargs
  from .base import Callback
label_desc: Use image node_id 5 for calling the ProgressBar obj's underlying member method code with example usage: obj._before_run(_) and returns: self, None