Initializing help system before first use

# pycall

Purpose
Call a Python object, e.g. a function, with optional input arguments and convert the result to a Mosel variable.
Synopsis
`procedure pycall(expr:string,result:array|set|list[, arg1...])`
`procedure pycallvoid(expr:string[, arg1...])`
`function pycallbool(expr:string[, arg1...]):boolean`
`function pycallint(expr:string[, arg1...]):integer`
`function pycallreal(expr:string[, arg1...]):real`
`function pycallstr(expr:string[, arg1...]):string`
`function pycalltext(expr:string[, arg1...]):text`
Arguments

expr | Global name of a callable Python object or Python expression that evaluates to a callable object
result | Result Mosel array, set or list
arg1, arg2,... | Optional input arguments for the object call

Example 1
The following example calls the Python print and max functions. The Mosel input arguments are automatically converted to Python objects and the return value of max is converted to a Mosel real.

    pycallvoid("print", "Python objects:", true, 1, 2.2, [3,4], {5})
    writeln("max: ", pycallreal("max", [1.1, 7.7, 4.4]))

Example 2
The following example uses pandas to compute the mean value of two Mosel arrays. The Mosel input arrays are passed as a single list of arrays with compatible array indices and are automatically converted to a single pandas DataFrame. The return value of the mean function is a pandas Series and its elements will be written to the Mosel Result array.

    declarations
      Input1, Input2, Result: array(range) of real
    end-declarations

    Input1 :: (0..4)[1, 2, 3, 4, 5]
    Input2 :: (0..4)[7, -1, -3, 1, 2]
    pyinitpandas
    pyexec("def mean(df, axis): return df.mean(axis)")
    pycall("mean", Result, [Input1, Input2], 1)
    writeln("mean: ", Result)

It is recommended to define small global wrapper functions like in this example, instead of calling functions or methods with the help of an expression like "pandas.Series.mean". The global function can be found directly without having to perform an expensive Python string evaluation to retrieve the callable object.
Further information
1. At first, the function interprets the expression string as a global Python object name and tries to access it by getting it from the attributes of the Python __main__ module. If this fails, the expression is evaluated by Python. Then the expression result object will be called with the optional input arguments. Finally, the result of the object call is stored in or returned as a Mosel variable. This is equivalent to the Python expression:

    expr(arg1, ...)

It is a fatal error if the expression cannot be evaluated or if the object call or the type conversion between Python and Mosel fails.
2. The first version of the pycall routine stores the result in an array, set or list. Its behavior is additive: it writes the new elements to the existing Mosel array, set or list without clearing previously existing elements. Use reset to manually clear an array, set or list before calling this function.
3. See the I/O Driver python Section for further details about type conversions.
4. Do not concatenate untrusted strings from the end user into the expr string. See Section Xpress Insight 5 configuration for more information.
Related topics
pyexec, pyget, Driver python
Module
python3
Parent Topic Procedures and functions

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