# LibOptions: globals, locals, has_changed and all that

To simplify parsing of options and handling of defaults, the Options class was created. It functions in the following way:

- Each module (or plugin) declares which options it will look for in the input: their name, type (string, int, double, array, etc.), and any default value they take.
- The input is parsed for these options, and defaults are assigned for those keywords not specified by the user.
- The c-side module or plugin can then query the Options object for the values associated with each keyword.
- The options will also be accessible py-side to the procedures that drive the modules. Array-type options are not available in python.

## Declaring Options

Each module needs to make itself known to the Options object, via a read_options function. For plugins, this routine is provided by the user in the plugin code. For native PSI4 modules, the entries need to be appended to the read_options code in psi4/src/bin/psi4/read_options.cc. An example of such a routine is

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    if (name == "MYMODULE"|| options.read_globals()) {
        /*- The amount of information printed
        to the output file -*/
        options.add_int("PRINT", 1);
        /*- Do save information to |mymodule__data_file| at the end of the computation? -*/
        options.add_bool("SAVE_INFO", true);
        /*- An array containing the number of doubly occupied orbitals per irrep
        (in :ref:`Cotton order <table:irrepOrdering>`) -*/
        options.add("DOCC", new ArrayType());
        /*- The factor by which the harmonic vibrational frequencies are multiplied to
        obtain an approximation to the fundamental vibrational frequencies -*/
        options.add_double("FREQUENCY_SCALE_FACTOR", 1.0);
        /*- The filename to which data is dumped. !expert -*/
        options.add_str("DATA_FILE", "data.dat");
        /*- The algorithm to use for the $\left<VV||VV\right>$ terms -*/
        options.add_str("AO_BASIS", "NONE", "NONE DISK DIRECT");
    }

In the above example, the following options are declared (in order):

- An integer called `PRINT` with a default value of 1.
- A boolean called `SAVE_INFO` with a default of true.
- An array called `DOCC`, no default is possible for this type.
- A double called `FREQUENCY_SCALE_FACTOR` with a default of 1.0.
- A string called `DATA_FILE`, with a default of “data.dat” and any possible value.
- A string called `AO_BASIS` with a default of “NONE”, and possible values of “NONE”, “DISK”, or “DIRECT”.

The purpose of the “if” statement in the above read_options function is the following. Suppose in an input file the user sets an option through the construct `set mymodule print 1` or through a `set mymodule {...}` block. The first thing to happen is a call to read_options with name set to “MYMODULE”. (Note that all user input is converted to upper case.) This call to read_options should tell the Options object only about those options expected by the module called “mymodule”; this prevents overlap of options between different modules.

Notice also that there’s a special comment immediately before the declaration of each keyword. You must provide these comments for any options you add as they will be automatically inserted into the user manual Providing a clear description will also help you to remember what the keywords do and how they’re used. The comments must live between the special comment delimiters. For options that most users shouldn’t need, add an expert flag to the comment. This will place these options in a separate section of the user manual.

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    /*- comment -*/
    options.add_ ...
    /*- comment !expert -*/
    options.add_ ...

As is apparent from the examples above, comments can span multiple lines (see `PRINT`), can refer to other options (through hyperlinks; see `SAVE_INFO`), can refer to sections of the manual (through hyperlinks; see `DOCC`), and can contain LaTeX notation (see `AO_BASIS`). (To get the LaTeX subscript command, use “@@” instead of “_”.)

See Best Practices for guidelines on naming options.

## What is has_changed ?

There are times when we need to know whether an option was provided by the user or if the defaults are being used. For this reason, the Options object stores a boolean has_changed value, in addition to the option value itself. A clarification of definition:

- [a] has_changed DOESN’T answer “Has option been changed by the user?”
- [b] has_changed DOESN’T answer “Is option now different from the default?”
- [c] has_changed DOES answer “Has option value been touched at all, by user or code?”

The above items notwithstanding, psi4 code should be written so that has_changed DOES effectively mean, “Has option been changed by the user?”. The way to do this is to isolate and nullify any changes to options made by the code, the difference between [a] and [c]. C-side, there is no concern since options are read but essentially never written-to within the modules.

Py-side is another matter since the driver’s role is to take terse instructions from the user and translate those into instructions to the C++ modules, usually through manipulation of options.

In order to preserve effective definition [a], the strategy for each python driver function is to query for the value of any option the function may want to change and for the current has_changed status (presumably reflecting whether the user has changed the value, as long as no preceding code has corrupted that definition). The python function then makes its changes to the option and runs any c-side modules with those changes. Finally, just before the function returns, the options are reset to the user’s value and has_changed status (which should now again reflect only whether the user has changed the value).

Warning

PUREAM is an exception in that its value and `has_changed()` value only reflect what the user has explicitly set. This keyword should not be queried to find out the current PUREAM state for the active basis; use instead, `psi4.MintsHelper().basisset().has_puream()`.

## Reading Options in Module

## Handling Options in Driver

This section is about the scopes of options and how best to handle them in the python driver. There are four groups of commands available. Options from the c-side Options object are accessible in the Python driver through four sets of commands.

- get
  - `get_global_option()`
  - `get_local_option()`
  - `get_option()`
- set
  - `set_global_option()`
  - `set_local_option()`
- has_changed
  - `has_global_option_changed()`
  - `has_local_option_changed()`
  - `has_option_changed()`
- revoke_changed
  - `revoke_global_option_changed()`
  - `revoke_local_option_changed()`

There’s a pattern here. Setting something, either a value (set) or a negative changed status (revoke_changed), can only be done for a specific scope, either global or local to the specified module. Querying, either a value (get) or a changed status (has_changed), can be done in the global scope, in a specified local scope, or in the context of “What will the specified module use?”.

Note

“Global” in the sense of the discussion has nothing to do with the globals section at the top of read_options.cc . That section is just a convenient place for options and associated values that are used by most, if not all, modules.

There are two primary purposes for interacting with options in the python driver.

- Preserving User Options (Enforcing definition [a] of has_changed)

  The first, less-interesting, use of retrieving user option values has been to preserve them so that they may be restored at the end after the procedure itself has clobbered them. By decoupling global_option and local_option commands, this can now be performed neatly by saving at the beginning the global and local values and the global and local has_changed values, then restoring them at the end. Below is an example of this procedure; don’t actually do this.

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      g_user_scftype = psi4.get_global_option('SCF_TYPE')
      l_user_scftype_scf = psi4.get_local_option('SCF', 'SCF_TYPE')
      bg_user_scftype = psi4.has_global_option_changed('SCF_TYPE')
      bl_user_scftype_scf = psi4.has_local_option_changed('SCF', 'SCF_TYPE')

      g_user_wfn = psi4.get_global_option('WFN')
      l_user_wfn = psi4.get_local_option('MP2', 'WFN')
      bg_user_wfn = psi4.has_global_option_changed('WFN')
      bl_user_wfn = psi4.has_local_option_changed('MP2', 'WFN')

      # body of function
      # scf_type and wfn are freely changed, LOCALLY
      # psi4.scf() and psi4.mp2() are run

      psi4.set_global_option('SCF_TYPE', g_user_scftype)
      if not bg_user_scftype:
          psi4.revoke_global_option_changed('SCF_TYPE')
      psi4.set_local_option('SCF', 'SCF_TYPE', l_user_scftype_scf)
      if not bl_user_scftype_scf:
          psi4.revoke_local_option_changed('SCF', 'SCF_TYPE')

      psi4.set_global_option('WFN', g_user_wfn)
      if not bg_user_wfn:
          psi4.revoke_global_option_changed('WFN')
      psi4.set_local_option('MP2', 'WFN', l_user_wfn_scf)
      if not bl_user_wfn_scf:
          psi4.revoke_local_option_changed('MP2', 'WFN')

  Instead of cluttering the driver with the above boilerplate, use an `OptionsState` object that stores values and has_changed values for each keyword and module pair given to it as arguments. At the end of the python function, these stored user settings can be restored.

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      optstash = OptionsState(
          ['SCF', 'SCF_TYPE'],
          ['MP2', 'WFN'],
          ['DF_BASIS_SCF'])

      # body of function
      # scf_type and wfn are freely changed, LOCALLY
      # puream and df_basis_scf are freely changed, GLOBALLY
      # psi4.scf() and psi4.mp2() are run

      optstash.restore()

  Note

  Some options (BASIS, BASIS-like, and PUREAM) should always be used globally (no module argument) with the OptionsState objects. Similarly, within the body of the function, they should always be queried and set globally. Same for FREEZE_CORE.

- Setting-Up Calculations

  The other types of options calls in python driver functions are (a) those to query what option value an upcoming c++ module is going to use (determined by user and defaults) and (b) those to set options to govern the course of a procedure. Finding out the intended option value for a molecule should employ the `get_option()` command (and `has_option_changed()` for has_changed), which (newly) requires a module for scope. (Previously, this command used the “active module”, which isn’t well-defined in the context of the python driver, and consequently, the command gave variable results, depending on whether a get_local/set_local command had been previously executed to define the active module.)

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      if (psi4.get_option('SCF', 'REFERENCE') == 'RHF'):
          psi4.set_local_option('SCF', 'REFERENCE', 'RKS')

  Setting of options in python should use the `set_local_option()` command. Using the local, rather than global, scope will ensure that the newly set option will be used by the module. Otherwise, if the python procedure set in the global scope and the user had happened to set that option in local scope, the local user option will take precedence against the programmer’s intent. (Anyone who has heard advice to “query local, set global” should forget that and follow the new scheme outlined here.)