In your class methods you may use
self.params interchangeably - they are actually the same object.
(see the 'Parameters' section in the docs: https://www.backtrader.com/docu/concepts/)
One need to distinguish between the
params class variable and the
self.p for that matter) instance variable.
params class variable (list of tuples or dict) defines the custom parameters and their default values that will be later used to "magically" create and populate the
self.p) instance variable
self.p) instance variable will have members for each tuple originally present in
params class variable but their value could be different depending of which parameters where passed during class instantiation ( or have their default values if no matching parameters were passed )
params class variable may be defined in classes that use MetaParams metaclass (or its derivatives) during their instantiation sequences (like Cerebro, Strategy, Analyzer, BrokerBase, Filter, Indicator, an so on and all their inherited classes ).
During the instantiation the meta-class "hijack" the instantiation sequence and creates additional instance variables (
self.params in particular, but also many others like 'datas', 'data0' and other aliazes for LineSeries inherited classes for example), intializing them in various ways. You may take a look at the
with_metaclass method in utils\py3.py for technical details of how this is implemented.
For example one of such meta-classes is
MetaParams class which is particularly responsible for creating the
self.params instance variables ( see its
donew method )
During your strategy (derived from
Strategy class which uses
MetaParams meta-class) instantiation process a new auto-generated class (let's call it X) will be used to hold the parameter values. For each tuple in the 'params' class variable there will be a member in
X class. The instance of this
X class will be created, where each member will get its value either from the parameters passed during the
Cerebro.addstrategy call or from the default value in the
params class variable tuples.
For example let's assume your strategy
params variable defined as:
class SMACrossOver(bt.Strategy): params = ( ('stake', 1), ('period', 30), )
and you add this strategy to Cerebro using:
The values for 'self.p' members inside your strategy will be:
self.p.stake == 1 # from default value in `params` class variable self.p.period == 15 # from parameters to cerebro.addstrategy