HOC accessing Python

This section describes how one can interact with Python from HOC code. For more information about the Python interface to NEURON, see the Python documentation.


nrnpython("any python statement")


Executes any python statement. Returns 1 on success; 0 if an exception was raised or if python support is not available.

In particular, python_available = nrnpython("") is 1 (true) if python support is available and 0 (false) if python support is not available.


nrnpython("import sys")
nrnpython("print sys.path")
nrnpython("a = [1,2,3]")
nrnpython("print a")
nrnpython("import hoc")
nrnpython("hoc.execute('print PI')")

class PythonObject

p = new PythonObject()


Accesses any python object. Almost equivalent to HocObject in the python world but because of some hoc syntax limitations, ie. hoc does not allow an object to be a callable function, and top level indices have different semantics, we sometimes need to use a special idiom, ie. the ‘_’ method. Strings and double numbers move back and forth between Python and Hoc (but Python integers, etc. become double values in Hoc, and when they get back to the Python world, they are doubles).

objref p
p = new PythonObject()
nrnpython("ev = lambda arg : eval(arg)") // interprets the string arg as an
                          //expression and returns the value
objref tup
print p.ev("3 + 4")       // prints 7
print p.ev("'hello' + 'world'") // prints helloworld
tup = p.ev("('xyz',2,3)") // tup is a PythonObject wrapping a Python tuple
print tup                 // prints PythonObject[1]
print tup._[2]            // the 2th tuple element is 3
print tup._[0]            // the 0th tuple element is xyz

nrnpython("import hoc")   // back in the Python world
nrnpython("h = hoc.HocObject()") // tup is a Python Tuple object
nrnpython("print h.tup")   // prints ('xyz', 2, 3)

Note that one needs the ‘_’ method, equivalent to ‘this’, because trying to get at an element through the built-in python method name via


gives the error “TypeError: tuple indices must be integers” since the Hoc 0 argument is a double 0.0 when it gets into Python. It is difficult to pass an integer to a Python function from the hoc world. The only time Hoc doubles appear as integers in Python, is when they are the value of an index. If the index is not an integer, e.g. a string, use the __getitem__ idiom.

objref p
p = new PythonObject()
nrnpython("ev = lambda arg : eval(arg)")
objref d
d = p.ev("{'one':1, 'two':2, 'three':3}")
print d.__getitem__("two")        // prints 2

objref dg
dg = d.__getitem__
print dg._("two")                // prints 2

To assign a value to a python variable that exists in a module use

nrnpython("a = 10")
p = new PythonObject()
p.a = 25
p.a = "hello"
p.a = new Vector(4)
nrnpython("b = []")
p.a = p.b