Memory Optimisation & const()

[Beormund]

If I have a lookup like so:

lookup = {
    'on': 1,
    'off': 0,
    '1': 1,
    '0': 0,
    'true': 1,
    'false': 0
}

How can I best utilise const() to optimise RAM use? Do I first have to declare _on = const('on') and then use _on as a dictionary key? Same with values?

With regards to long strings, would there be any advantage to declaring XML strings like so:

_GETBINARYSTATE_SOAP = const("""<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/" s:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/">
  <s:Body>
    <u:GetBinaryStateResponse xmlns:u="urn:Belkin:service:basicevent:1">
      <BinaryState>%(state_relay)s</BinaryState>
    </u:GetBinaryStateResponse>
  </s:Body>
</s:Envelope>
""")

And is this the correct way to declare a const tuple?

_abbr = const(('last', 'first', 'second', 'third', 'fourth'))

[peterhinch]

The const declaration only works when used in global scope e.g.

_t = const((1, 2, 3))

Without the const declaration the compiler would generate a global variable _t with the value (1, 2, 3). For any reference to _t code would be generated to look up the variable _t. With the const declaration, a reference such as

foo(_t)

would be compiled as

foo((1, 2, 3))

In other words, _t would use no RAM because the symbol is expanded at compile time; the time used to access the global would also be saved. However the constant tuple (1, 2, 3) has to be stored somewhere: as a const it is stored once in code space. This will be either RAM or flash depending on whether the code is frozen as bytecode.