Package ZenRRD :: Module rpn :: Class FSM

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Class FSM

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This is a Finite State Machine (FSM).

Instance Methods

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__init__(self, initial_state, something)
This creates the FSM.

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reset(self)
This sets the current_state to the initial_state and sets input_symbol to None.

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add_transition(self, input_symbol, state, action, next_state)
This adds a transition that associates (input_symbol, current_state) --> (action, next_state) The action may be set to None in which case the process() method will ignore the action and only set the next_state.

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add_transition_list(self, list_input_symbols, state, action, next_state)
This adds the same transition for lots of different input symbols.

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add_transition_any(self, state, action, next_state)
This adds a transition that associates (current_state) --> (action, next_state) The process() method checks these associations if it cannot first find a match of an (input_symbol, current_state).

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set_default_transition(self, action, next_state)
This sets the default transition.

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get_transition(self, input_symbol, state)
This returns (action, next state) given an input_symbol and state.

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process(self, input_symbol)
This is the main method that you call to process input.

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process_list(self, s)
This takes a list and sends each element to process().

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Method Details

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init(self, initial_state, something)
(Constructor)

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This creates the FSM. You set the initial state here. The "something" attribute is any object that you want to pass along to the action functions. It is not used by the FSM. For parsing you would typically pass a list to be used as a stack.

reset(self)

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This sets the current_state to the initial_state and sets input_symbol to None. The initial state was set by the constructor __init__().

add_transition(self, input_symbol, state, action, next_state)

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This adds a transition that associates
        (input_symbol, current_state) --> (action, next_state)
The action may be set to None in which case the process() method 
will ignore the action and only set the next_state.
   
You can also set transitions for a list of symbols by using
add_transition_list().

add_transition_list(self, list_input_symbols, state, action, next_state)

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This adds the same transition for lots of different input symbols. You can pass a list or a string. Note that it is handy to use string.digits, string.whitespace, string.letters, etc. to add transitions that match character classes.

set_default_transition(self, action, next_state)

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This sets the default transition. This defines an action and next_state if the FSM cannot find the input symbol and the current state in the transition list and if the FSM cannot find the current_state in the transition_any list. This is useful for catching errors and undefined states.

The default transition can be removed by setting the attribute default_transition to None.

get_transition(self, input_symbol, state)

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This returns (action, next state) given an input_symbol and state.
This leaves the FSM unchanged. This does not update the current state 
nor does it trigger the output action. Normally you do not call 
this method. It is called by process().

The sequence of steps to check for a defined transition goes from 
the most specific to the least specific. 
1. Check state_transitions[] that match (input_symbol, state)
2. Check state_transitions_any[] that match (state)
   In other words, match a specific state and ANY input_symbol.
3. Check if the default_transition is defined.
   This catches any input_symbol and any state.
   This is a handler for errors, undefined states, or defaults.
4. No transition was defined. If we get here then raise an exception.

process(self, input_symbol)

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This is the main method that you call to process input. This may cause the FSM to change state and call an action. This method calls get_transition() to find the action and next_state associated with the input_symbol and current_state. If the action is None then the action is not called and only the current state is changed. This method processes one input symbol. You can process a list of symbols (or a string) by calling process_list().

process_list(self, s)

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This takes a list and sends each element to process(). The list may be a string.

Class FSM

__init__(self, initial_state, something) (Constructor)

reset(self)

add_transition(self, input_symbol, state, action, next_state)

add_transition_list(self, list_input_symbols, state, action, next_state)

set_default_transition(self, action, next_state)

get_transition(self, input_symbol, state)

process(self, input_symbol)

process_list(self, s)

init(self, initial_state, something)
(Constructor)