Problem importing class from another Python file

Question

I'm very new to Python, I'm having trouble wrapping my head around some aspects of using Python.

I'm writing code to control a couple of instruments using pyvisa. For simplicity, I'm only showing one instrument here (a Thor Labs power meter). If I put all my code together into one file I can get things to work. The code below first searches for available instruments, and defines the instrument ID (ThorID):

import pyvisa
rm = pyvisa.ResourceManager()  # Set up resource manager
resource_list = rm.list_resources()  # Create a list of available instruments

# Thor Labs power meters should contain this string
Thor_str = "0x1313::0x8079"
# Search for a Thor Labs meter in the resource list
Thor = [i for i, s in enumerate(resource_list) if Thor_str in s]

if len(Thor) == 0:
   print("No Thor Labs power meter detected")
   ThorID = ""
else:
   print("Thor Labs power meter detected")
   ThorID = resource_list[Thor[0]]  # This is the Thor Labs VISA ID

Then I define the class powermeter

class powermeter:
"Power meter class. Contains parameter values and commands."

def __init__(self, wavelength=450, meter_range=200e-3, id=ThorID):
    """ Define default parameter values """
    self.wavelength = wavelength  # nm
    self.meter_range = meter_range  # W
    self.ID = id  # Meter's VISA ID
    self.name = ""  # Meter instance name

def connect(self):
    """Open a connection to the power meter"""
    self.name = rm.open_resource(self.ID)

def set_wavelength(self, wavelength):
    """Set the power meter wavelength (nm)"""
    self.name.write("CORR:WAV " + str(wavelength))

def set_range(self, meter_range):
    """Set the power meter range (W)"""
    self.name.write("POW:DC:RANG " + str(meter_range))

def read(self):
    """Read the power meter output (mW)"""
    return float(self.name.query("READ?"))*1000

Final 3 lines opens a connection to the meter and sets the wavelength (to 400 nm). I can confirm all this code together in one .py file works.

enter pm = powermeter()  # Create instance of powermeter
pm.connect()  # Connect to this power meter
pm.set_wavelength(400)  # Set wavelength to 400 nm

I run into lots of trouble trying to split the Class off into a separate .py file. I want to do this to try to keep my code tidier...eventually I'll add in a GUI and another class and I don't want one huge cumbersome monolith of text.

My naive first attempt has this as my main code, with the class as written above saved in PM_class.py:

import pyvisa

rm = pyvisa.ResourceManager()  # Set up resource manager
resource_list = rm.list_resources()  # Create a list of available instruments

# Thor Labs power meters should contain this string
Thor_str = "0x1313::0x8079"
# Search for a Thor Labs meter in the resource list
Thor = [i for i, s in enumerate(resource_list) if Thor_str in s]

if len(Thor) == 0:
    print("No Thor Labs power meter detected")
    ThorID = ""
else:
    print("Thor Labs power meter detected")
    ThorID = resource_list[Thor[0]]  # This is the Thor Labs VISA ID

from PM_class.py import powermeter

But I get an error in PM_class.py - NameError: name 'ThorID' is not defined

I don't understand that, because ThorID is clearly defined in the line before I try to import the Class?

Any advice on how to approach this would be very welcome!

Answer 1

class powermeter is in another module, in this string

def __init__(self, wavelength=450, meter_range=200e-3, id=ThorID):

you try to init it with var ThorID but the class itself is encapsulated and cannot read this var from your current module. Try to define ThorID variable in PM_class.py or use

def __init__(self, wavelength=450, meter_range=200e-3, id=None):

and pass id when initialising the class

pmeter=powermeter(id=ThorID)