Tips and Examples

Scripting > Tips and Examples

Tips from the developers

Even with correct syntax, a working script can become bug-prone and difficult for users to update. Here we document some tips to help keep your scripts working as expected and easy to modify in the future.

Argument ordering

As this is Python, genie_python conforms to the standard pattern of calling Python functions. The arguments to the function are contained within brackets and the variables passed in as a comma-separated list. Ordering is important but can be overridden by using named variables, for instance the following are all correct and equivalent:

g.change_beamline_pars("PAR1",1)
g.change_beamline_pars(name="PAR1",value=1)
g.change_beamline_pars(value=1,name="PAR1")
g.change_beamline_pars("PAR1",value=1)

In the last example, named and unnamed variables are mixed. Unnamed variables must precede named variables. The following examples are not valid

g.change_beamline_pars(name="PAR1",1) # Named variable before unnamed
g.change_beamline_pars(1,"PAR1") # Cannot change order of unnamed variables

Using named variables can be very useful in avoiding mistakes. For instance, getting the order of high and low limits the wrong way round. For instance this example:

g.change_monitor(1,10,0)

is wrong and wouldn't work. Instead, we could have written:

g.change_monitor(1,high=10,low=0)

which would have worked and makes it clear for whoever comes to edit the code in future (hint: that person might be you!).

Making Errors Standout in Python Console

Python console will display most output in black this is from the standard out but will display error information in red. Error information should be written to standard error using sys.stderr.write("errorn").

Some examples

Sequentially sets a position (e.g. sample changer) and waits for a fixed number of uamps

from genie_python import genie as g, BLOCK_NAMES as b

def loop_over_samples(block_name, position_names, charge_to_wait_for):
    """
    Begins a run. Loops over a set of sample positions. Ends the run

    :param block_name: The name of the block containing the sample position
    :param position_names: A collection of the positions to loop over
    :param uamps_to_wait_for: The number of uamps to wait for between samples
    """
    g.begin()
    total_charge_to_wait_for = 0
    for position_name in position_names:
        print("Moving sample changer to position: {0}".format(position_name))
        g.cset(block_name, position_name)
        total_charge_to_wait_for += charge_to_wait_for
        g.waitfor_uamps(total_charge_to_wait_for)
    g.end()

Reads the value of a block and moves it through a series of intervals to a new values taking a fixed time

from genie_python import genie as g, BLOCK_NAMES as b

def ramp(time, block_name, steps=100, final_value=0.0):
    """
    Moves a block between an initial and target value over a sequence of steps, waiting a fixed time at each step.

    :param time: The overall time to take to ramp. Time will be split evenly between steps
    :param nsteps: The number of steps to take during the ramp
    :param block_name: The name of the block whose value to set
    :param final_value: The value we want to ramp to
    """

    initial_value = g.cget(block_name)['value']
    if initial_value is None:
        print("Unable to determine temperature from block {0}. No action will be taken".format(block_name))
        return

    def values_match(v1, v2):
        tolerance = 0.001
        small = 1.0e-20
        return 2*abs(v1-v2)/(abs(v1)+abs(v2)+small) < tolerance

    step_duration = time/steps
    for i in range(1, steps+1):
        step_value = initial_value + (final_value - initial_value)/steps*i
        g.cset(block_name, step_value)
        print("Ramping to: {0} Target value: {1}".format(step_value, final_value))
        g.waitfor_time(seconds=step_duration)
        new_block_value = g.cget(block_name)['value']
        if not values_match(new_block_value, step_value):
            print("WARNING: The current value for block {0}, {1}, does not match the target value of {2}".format(block_name, new_block_value, step_value))

    print("Ramp complete")