Usage

Initialization Code

The initialization code iPyInit in MQL4/Include/OTMql4/OTPy27.mqh initializes the Python environment. This should be called from your OnInit() function. It is safe to call it a second time; subsequent calls will just be ignored.

It has an integer return value, and should return 0. A return value of -1 is a panic: remove the expert if it requires Python.

It calls the compiled PyInitialize() and then imports some standard system modules. Then it prepends the sys.path with the directory MQL4/Python, which should have been created when you installed OTMql4Py. In that directory should be a (possibly empty) file __init__.py, so that you can import modules found in that directory into Python.

The intialization code will import the module OTMql427 found in that directory to give some added functionality. If it can't import the module OTMql427 it will signal a panic by returning -1: you should fix the problem before going any futher.

Global Variables

The iPyInit initialization creates a temporary global veriable called fPythonUsers and increments it by one each time it is called. vPyDeInit decrements it by one each time it is called, and if fPythonUsers is zero, then it calls OTMql427.vPyDeInit to unload the Python interpreter. fPythonUsers should always be equal to the number of charts and scripts Python is being used on. Unfortunately, if your recompile your expert while Python is loaded, then Mt4 will deinit your expert and re-init the expert. If you only had one chart using Python (fPythonUsers=1), then this will unload Python when Mt4 deinits the expert, and when Mt4 re-inits the Python, it will fail to initialize the py27.dll properly. If Mt4 OnInit had a required reason argument the way OnDeinit does, we could work around this, but it doesn't. Suggestions welcome...

The iPyInit initialization also creates a persistent global veriable called "fDebugLevel" which is used by the logging code, and it ranges from 0 to 5: 0 : quiet, 1 : +errors, 2 : +warnings, 3 +info, 4 : +debug, 5 : +trace.

Added Python Functionality

In many cases, you should use uPySafeEval to evaluate a python expression that will evaluate to a string and return its value. It calls OTMql427.sPySafeEval in Python which wraps the code to be evaluated in a try:/except: clause and catches the error. If there's an error, the error is returned as a string, prepended with ERROR: .

In the caller you should have something like:

    if (StringFind(uSource, "ERROR:", 0) == 0) {
      Print("Error in Python evaluating: " + uSource + "\n" + res);
      <do something as a result of the failure>
    }

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