Designing A Remote Laboratory With PSLab: execution of function strings
In the previous blog post, we introduced the concept of a ‘remote laboratory’, which would enable users to access the various features of the PSLab via the internet. Many aspects of the project were worked upon, which also involved creation of a web-app using EmberJS that enables users to create accounts , sign in, and prepare Python programs to be sent to the server for execution. A backend APi server based on Python-flask was also developed to handle these tasks, and maintain a postgresql database using sqlalchemy .
The following screencast shows the basic look and feel of the proposed remote lab running in a web browser.
This blog post will deal with implementing a way for the remote user to submit a simple function string, such as get_voltage(‘CH1’), and retrieve the results from the server.
There are three parts to this:
- Creating a dictionary of the functions available in the sciencelab instance. The user will only be allowed access to these functions remotely, and we may protect some functions as the initialization and destruction routines by blocking them from the remote user
- Creating an API method to receive a form containing the function string, execute the corresponding function from the dictionary, and reply with JSON data
- Testing the API using the postman chrome extension
Creating a dictionary of functions :
The function dictionary maps function names against references to the actual functions from an instance of PSL.sciencelab . A simple dictionary containing just the get_voltage function can be generated in the following way:
from PSL import sciencelab I=sciencelab.connect() functionList = {'get_voltage':I.get_voltage}
This dictionary is then used with the eval method in order to evaluate a function string:
result = eval('get_voltage('CH1')',functionList) print (result) 0.0012
A more efficient way to create this list is to use the inspect module, and automatically extract all the available methods into a dictionary
functionList = {} for a in dir(I): attr = getattr(I, a) if inspect.ismethod(attr) and a!='__init__': functionList[a] = attr
In the above, we have made a dictionary of all the methods except __init__
This approach can also be easily extrapolated to automatically generate a dictionary for inline documentation strings which can then be passed on to the web app.
Creating an API method to execute submitted function strings
We create an API method that accepts a form containing the function string and option that specifies if the returned value is to be formatted as a string or JSON data. A special case arises for numpy arrays which cannot be directly converted to JSON, and the toList function must first be used for them.
@app.route('/evalFunctionString',methods=['POST']) def evalFunctionString(): if session.get('user'): _stringify=False try: _user = session.get('user')[1] _fn = request.form['function'] _stringify = request.form.get('stringify',False) res = eval(_fn,functionList) except Exception as e: res = str(e) #dump string if requested. Otherwise json array if _stringify: return json.dumps({'status':True,'result':str(res),'stringified':True}) else: #Try to simply convert the results to json try: return json.dumps({'status':True,'result':res,'stringified':False}) # If that didn't work, it's due to the result containing numpy arrays. except Exception as e: #try to convert the numpy arrays to json using the .toList() function try: return json.dumps({'status':True,'result':np.array(res).tolist(),'stringified':False}) #And if nothing works, return the string except Exception as e: print( 'string return',str(e)) return json.dumps({'status':True,'result':str(res),'stringified':True}) else: return json.dumps({'status':False,'result':'unauthorized access','message':'Unauthorized access'})
Testing the API using Postman
The postman chrome extension allows users to submit forms to API servers, and view the raw results. It supports various encodings, and is quite handy for testing purposes.Before executing these via the evalFunctionString method, user credentials must first be submitted to the validateLogin method for authentication purposes.
Here are screenshots of the test results from a ‘get_voltage(‘CH1’)’ and ‘capture1(‘CH1’,20,1)’ function executed remotely via postman.
Our next steps will be to implement the dialog box in the frontend that will allow users to quickly type in function strings, and fetch the resultant data
Resources:
- Introduction to remote access for PSLab
- Postman chrome extension and API testing tool
- Python-Flask : An extensible web microframework for building webapps with Python
- EmberJS : A framework for creating web applications
