class_docstr: Calculates a commission for a transaction based on a per trade cost.
  
  Parameters
  ----------
  cost : float, optional
      The flat amount of commissions paid per equity trade.
class_docstr_tok: ["Calculates","a","commission","for","a","transaction","based","on","a","per","trade","cost",".","Parameters","--","--","--","--","--","cost",":","float",",","optional","The","flat","amount","of","commissions","paid","per","equity","trade","."]
code: def __repr__(self):
      return "{class_name}(cost_per_trade={cost})".format(
          class_name=self.__class__.__name__,
          cost=self.cost,
      )
  
imports: from abc import abstractmethod
  from collections import defaultdict
  from six import with_metaclass
  from toolz import merge
  from catalyst.assets import Equity, Future
  from catalyst.finance.constants import FUTURE_EXCHANGE_FEES_BY_SYMBOL
  from catalyst.finance.shared import AllowedAssetMarker, FinancialModelMeta
  from catalyst.utils.dummy import DummyMapping
label_desc: Use image node_id 2 for calling the PerTrade obj's underlying member method code with example usage: obj.__repr__() and returns: str