Template:Language - Python Write Code

From Phidgets Support

Write Code

By following the instructions for your operating system and compiler above, you now have working examples and a project that is configured. This teaching section will help you understand how the examples were written so you can start writing your own code.


Remember: your main reference for writing Python code will be the Phidget22 API Manual and the example code.

Step One: Create and Address

You will need to declare your Phidget object in your code. For example, we can declare a digital input object like this: 

ch = DigitalInput()

Next, we can address which Phidget we want to connect to by setting parameters such as DeviceSerialNumber. 

ch.setDeviceSerialNumber(496911)

Although we are not including it on this page, you should handle exceptions of all Phidget functions. Here is an example of the previous code with error handling: 

try:
  ch = DigitalInput()	
except RuntimeError as e:
  print("Runtime Error: %s" % e.message) 
try:
  ch.setDeviceSerialNumber(496911)
except PhidgetException as e:
  print (“Phidget Exception %i: %s” % (e.code, e.details))

Step Two: Open and Wait for Attachment

After we have specified which Phidget to connect to, we can open the Phidget object like this: 

ch.openWaitForAttachment(5000)

To use a Phidget, it must be plugged in (attached). We can handle this by calling openWaitForAttachment, which will block indefinitely until a connection is made, or until the timeout value is exceeded. Simply calling open does not guarantee you can use the Phidget immediately.

Alternately, you could verify the device is attached by using event driven programming and tracking the attach events.

To use events to handle attachments, we have to modify our code slightly: 

ch = DigitalInput()	
ch.setOnAttachHandler(onAttachHandler)
ch.openWaitForAttachment(5000)

Next, we have to declare the function that will be called when an attach event is fired - in this case the function onAttachHandler will be called: 

def onAttachHandler(e):
    print("Phidget attached!")

We recommend using this attach handler to set any initialization parameters for the channel such as DataInterval and ChangeTrigger from within the AttachHandler, so the parameters are set as soon as the device becomes available.

Step Three: Do Things with the Phidget

We recommend the use of event driven programming when working with Phidgets. In a similar way to handling an attach event as described above, we can also add an event handler for a state change event: 

ch = DigitalInput()	
ch.setOnAttachHandler(onAttachHandler)
ch.setOnStateChangeHandler(onStateChangeHandler)
ch.openWaitForAttachment(5000)

This code will connect a function to an event. In this case, the onStateChangeHandler function will be called when there has been a change to the channel's input. Next, we need to create the onStateChangeHandler function: 

def onStateChangeHandler(self, state):
    print("State %f" % state)
    return 0

If you are using multiple Phidgets in your program, check out our page on Using Multiple Phidgets for information on how to properly address them and use them in events.

If events do not suit your needs, you can also poll the device directly for data using code like this: 

state = ch.getState()
print("State %f" % state)

Important Note: There will be a period of time between the attachment of a Phidget sensor and the availability of the first data from the device. Any attempts to get this data before it is ready will result in an exception. See more information on this on our page for Unknown Values.

Enumerations

Some Phidget devices have functions that deal with specific predefined values called enumerations. Enumerations commonly provide readable names to a set of numbered options.

Enumerations with Phidgets in Python will take the form of EnumerationType.ENUMERATION_NAME.

For example, specifying a SensorType to use the 1142 for a voltage input would look like: 

VoltageSensorType.SENSOR_TYPE_1142

and specifying a K-Type thermocouple for a temperature sensor would be: 

ThermocoupleType.THERMOCOUPLE_TYPE_K

The Phidget error code for timing out could be specified as: 

ErrorCode.EPHIDGET_TIMEOUT

You can find the Enumeration Type under the Enumerations section of the Phidget22 API for your device, and the Enumeration Name in the drop-down list within.

Step Four: Close and Delete

At the end of your program, be sure to close and delete your device: 

ch.close()
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