Working with a pool of input parameters

Parampool's main focus is on scientific applications with lots of input data of different type, organized in a hierarchical tree fashion. The various input parameters are defined in terms of a pool. The pool can be defined as a nested list or through a function application programming interface (known as an API, and here consisting of calls functionality in the parampool.pool package).

To exemplify the use of pools, we apply the compute_motion_and_forces function (from the section More input parameters and results):

def compute_motion_and_forces0(

Let us organize the 10 input parameters into four subpools. At the top level we need a subpool, usually called "Main pool" or named after the application. Each subpool is here specified with a logical name of each parameter and the corresponding variable name in the compute function:

With a pool we can give the parameters more readable logical names (not restricted to a valid variable name in Python), but we can also specify a lot other properties too, as will be explained.

: Parameter names must be unique! The generated Flask or Django code has a class (in or for Flask and Django, respectively) where each parameter name is transformed to a static class variable. Such code requires each parameter to have a unique name. (Using variable names that merge the parameter name with its subpool path would solve this problem.)

A pool is a hierarchical tree structure with subpools and data items, where each data item describes an input parameter in the problem. The task now is to make a Python specification of the of subpools and data items in the pool tree.

Specifying a pool as a list

The pool tree can be specified as a list of lists, strings, and dictionaries. Each list represents a subpool, each string the name of the subpool, and each dict is a data item. The pool must be return from some function, hereafter called the pool function. In our case, the pool function goes as follows:

def pool_definition_list():
    """Create and return pool defined through a nested list."""
    pool = [
        'Main', [
            'Initial motion data', [
                dict(name='Initial velocity', default=5.0),
                dict(name='Initial angle', default=45,
                     widget='range', minmax=[0,90], range_step=1),
                dict(name=r'Spinrate', default=50, widget='float',
            'Body and environment data', [
                dict(name='Wind velocity', default=0.0,
                     help='Wind velocity in positive x direction.',
                     minmax=[-50, 50], number_step=0.5,
                     widget='float', str2type=float),
                dict(name='Mass', default=0.1, unit='kg',
                     validate=lambda data_item, value: value > 0,
                     help='Mass of body.'),
                dict(name='Radius', default=0.11, unit='m',
                     help='Radius of spherical body.'),
            'Numerical parameters', [
                dict(name='Method', default='RK4',
                     options=['RK4', 'RK2', 'ForwardEuler'],
                     help='Numerical solution method.'),
                dict(name='Time step', default=None,
                     widget='textline', unit='s'),
            'Plot parameters', [
                dict(name='Plot simplified motion', default=True,
                     help='Plot motion without drag+lift forces.'),
                dict(name='New plot', default=True,
                     help='Erase all old curves.'),
    from parampool.pool.UI import listtree2Pool
    pool = listtree2Pool(pool)
    return pool

Actually, the pool function must return a parampool.pool.Pool object, so after the definition of the pool tree as a list we must make the shown conversion from a list to a Pool object via the listtree2Pool function.

Attributes in data items

Each data item has a name and preferably a default value, as in the case of "Initial velocity". More attributes can be added:

Check that default values are real numbers. If a default value is set to 5, Parampool will interpret this as an integer and let string2type be int and force all input to be converted to integers. Normally, you want input to be real, so check that the default value is 5.0 unless the pool item is really meant to be an integer.

The compute function

When working with pools, the compute function is allowed to take only one argument called pool. This object is used to extract input data. Basically, the value of any data item my parameter in the pool is extracted by

variable = pool.get_value('my parameter')

In case multiple data items have the same name, a sufficient part of the subpool path must be given, e.g.,

variable = pool.get_value('My Subpool1/my parameter')

Our specific compute function is a wrapper for compute_motion_and_forces:

def compute_motion_and_forces_with_pool(pool):
    initial_velocity = pool.get_value('Initial velocity')
    initial_angle = pool.get_value('Initial angle')
    spinrate = pool.get_value('Spinrate')
    w = pool.get_value('Wind velocity')
    m = pool.get_value('Mass')
    R = pool.get_value('Radius')
    method = pool.get_value('Method')
    dt = pool.get_value('Time step')
    plot_simplified_motion = pool.get_value('Plot simplified motion')
    new_plot = pool.get_value('New plot')
    return compute_motion_and_forces(
        initial_velocity, initial_angle, spinrate, w,
        m, R, method, dt, plot_simplified_motion,

The assumption is that the pool object provides enough input data for the compute function. If this assumption does not hold and extra information is needed, one can simply make a class, store extra data as attributes in the class, and let the compute function be a method in the class.

A command-line and file interface

Having defined a pool, it is trivial to get a command-line interface in the application. Just write

from parampool.pool.UI import set_values_from_command_line
pool = set_values_from_command_line(pool)

Now pool has values loaded from the command line. The name of the command-line options follow the names in the pool, but with underscore replacing whitespace: --Initial_motion_data/Initial_angle. However, in this case just writing --Initial_angle also works since it is a unique name in the pool tree, and then we do not need the complete path with the subpool name.

One can also read data from a file with syntax

subpool Initial motion data
  Initial angle = 45.5        # small perturbation
  Spinrate = 20
subpool Body and environment data
  Wind velocity = -10  ! m/s  # units appear after ! (before #)

Data from the file is loaded into the pool by

from parampool.pool.UI import set_defaults_from_file
pool = set_defaults_from_file(pool)

To activate reading from file mydat.dat, one must supply the command-line arguments --poolfile mydat.dat.

Tip: autogenerate the file with default data. The function write_poolfile(pool) in parampool.pool.UI writes the current pool data to a file with the right syntax. This is a simple way of getting the complete pool in the file.

Often, an application will first load default values from file, then from the command line, and finally launch the graphical web interface for enabling interactive setting of values in the pool system. Automatic generation of such interactive web interfaces constitutes the next topic. The lines above for loading parameters from file and command line are automatically generated when a web interface is requested (see also comments in the section Loading parameters from file and the command line).

A web-based user interface

With a pool function and a compute function at hand, it remains to make a new directory, copy the module(s) containing the pool function and compute function to this directory, and write a file with the content

from parampool.generator.flask import generate
from compute import compute_motion_and_forces_with_pool, \


The generate function will now use the information in the pool (and not the arguments in the compute function!) to generate a flexible user interface. Note that an Internet connection is required. After running

Terminal> python

several Flask files and directories are generated (,, templates, static, and a simple clean-up script The user interface is started by

Terminal> python

Open the URL in a web broswer to see an interface as the one in Figure 7.

Figure 7: Web interface in closed form.

Operating the graphical web interface

The pool tree is mapped onto a visual structure often used for directory trees. The look and feel resemble that of the Windows Explorer application in the Windows operating system.

Clicking on open all at the top of the user interface expands all subpools so that all parameters (data items) become visible. Figure 8 displays the result in the Opera browser. Note that in this type of user interface, the name of each data item is automatically typeset in LaTeX and inserted as a picture (the utility is used).

Figure 8: Web interface in fully expanded form.

The following technical points must be mentioned.

  1. A plain float or integer value gives a textline widget, while if a minmax range is specified, a float or integer widget (so-called HTML5 number field) is chosen.
  2. Data items whose widgets are specified as float or integer, or where this is implied because str2type is float or int, or the default value is a float or int and the minmax attribute is assign, are shown using the HTML5 input field called number. This is recogned by the small (and not so useful) arrows that can be used to adjust the number, but usually typing in the number manually is faster and more precise. An extra attribute, number_step controls the stepping when clicking on the arrows and also the allowed precision of a typed number (same as number_step, which by default is 0.001).
  3. When the widget is range, an HTML5 range field is used, which is usually rendered as a slider in browsers. The slider gets by default 100 steps (can be changed or specified individually for any data item).
  4. With the select widget we get a pull-down menu with the different options.
  5. Any data item whose default value is True or False maps directly to a checkbox for boolean parameters.
  6. Any data item with unit specified maps to an ordinary text field, since input consists of a number with an optional text for the unit. That is, if we choose to set unit='m/s' for the "Initial velocity" data item, the input field will not the an HTML5 number field, but a standard HTML text field.
  7. The names of the data items are typeset in LaTeX and shown as PNG images. This means that data item names may contain mathematical expressions: Spinrate $\omega$ for instance.
Warning. The HTML5 number field is rendered differently in different browsers. This can lead to strange layout of the input fields. In such cases it is recommended to avoid the HTML5 number field. This is easiest accomplished by explicitly specifying widget to be textline. This is also the default widget type if you equip the number with a unit or do not specify any widget, just a float or integer default value.

We can try out the interface:

  1. Set "Initial velocity" to 8.
  2. Move the slider for "Initial angle" to 55.
  3. Add a positive "Wind velocity" of -3.
  4. Specify "Mass"as the text 0.1*1000 g (i.e., we use g rather than the default kg as unit, but the value is still 0.1 kg).
  5. Choose RK2 for "Method".
  6. Set "Time step" to 0.12.
  7. Uncheck the "Plot simplified motion" boolean value.
  8. Hold the mouse pointer over the "Wind velocity" field to see the help string. Then point the mouse to "Mass" input field and the specified unit pops up. A combination of help and unit information is showed if both are given in the data item definition.
You should see something like Figure 9.

Figure 9: Web interface with input parameters filled out.

Now, press the Compute button. Figure 10 shows the resulting response. You can now play around and click the checkbox for Plot simplified motion and the recompute to see the effects of wind against the motion, drag, and lift (which are substantial in this example).

Figure 10: Web interface with input and results.

Detection of wrong input

Text in a number field. Write abc in the "Initial velocity" field and press the Compute button. The error message "Please enter a number" pops up.

Failure of user-provided validate function. Give a negative value for "Mass". The "Mass"data item has a validation function provided by us. A False value returned from this function gives rise to a DataItemValueError shown in the browser. It reads here

Mass = -0.1: validate function <lambda> claims invalid value.

Failure of converting string to right type. Write abc for "Radius". This is a text field so any text is in principle valid, but Parampool raises a TypeError with the message

could not apply str2type=<type 'float'> to value abc <type 'str'>

Failure in the compute function. Give a list [0.1, 0.2] for "Time step". Since the default is None, which causes str2type=eval, any Python expression is accepted in the interface, but the compute function used in our example in this tutorial will raise a TypeError because float(dt) fails when dt is a list. One could think of providing a tailored str2type function in this case:

def convert_time_step(value):
    # Value must be None or a float
    if value == 'None':
        return None
            return float(value)
        except TypeError:
            raise TypeError(
	    'Time step: could not convert "%s" to float, '
	    'must be None or float' % value)

Setting str2type=convert_time_step for the "Time step" data item gives an informative error message if the answer is not as expected: None or a floating-point number.

Loading parameters from file and the command line

Parameters can be assigned default values in a file and then other values on the command line, see the section A command-line and file interface, before the web GUI is offered to the user. When autogenerating the web interface, the magic lines from the section A command-line and file interface are automatically inserted in the file (for Flask or for Django). This means that when starting python we may add --poolfile name and any set of command-line options for setting individual parameters. This makes it easy to control which default values that will appear in the web GUI.

Common trouble: "Address already in use"

Sometimes, after much trial and error with developing a graphical user interface, one gets an error message that the IP address is already in use. To recover from this problem, run the lsof program to see which program that applies the 5000 port (Flask runs its server on, which means that it uses the 5000 port). Find the PID of the program that occupies the port and force abortion of that program:

Terminal> lsof -i :5000
python  48824  hpl    3u  IPv4 1128848      0t0  TCP ...
Terminal> kill -9 48824

Now you can restart the application.

Specifying a pool using an API

Instead of listing all the entries in the pool tree as strings, lists, and dicts in a nested data structure, you can use the Application Programming Interface (API) of the parampool.pool package. The pool defined above is alternatively programmed like this using the API:

def pool_definition_api():
    """Create and return pool using the parampool.pool API."""
    from parampool.pool.Pool import Pool
    pool = Pool()
    # Go to a subpool, but create it if it does not exist
    pool.subpool('Main pool')
    pool.subpool('Initial motion data')
    # Define data items for the current subpool
        name='Initial velocity', default=5.0)
        name='Initial angle', default=45,
        widget='range', minmax=[0,90])
        name='Spinrate', default=50, widget='float', unit='1/s')

    # Move to (and create) another subpool, as in a file tree
    pool.subpool('../Body and environment data')
    # Add data items for the current subpool
        name='Wind velocity', default=0.0,
        help='Wind velocity in positive x direction.',
        minmax=[-50, 50], number_step=0.5,
        widget='float', str2type=float)
        name='Mass', default=0.1, unit='kg',
        validate=lambda data_item, value: value > 0,
        help='Mass of body.')
        name='Radius', default=0.11, unit='m',
        help='Radius of spherical body.')

    pool.subpool('../Numerical parameters')
        name='Method', default='RK4',
        options=['RK4', 'RK2', 'ForwardEuler'],
        help='Numerical solution method.')
        name='Time step', default=None,
        widget='textline', unit='s', str2type=convert_time_step)

    pool.subpool('../Plot parameters')
        name='Plot simplified motion', default=True,
        help='Plot motion without drag+lift forces.')
        name='New plot', default=True,
        help='Erase all old curves.')
    return pool

The API is in many ways easier to use than the nested data structure with lists, strings, and dicts. The API resembles moving around in a file tree. The rules are simple:

The look and functionality of this GUI (found in the flask_pool2 directory) are the same as in the previous one (found in the flask_pool1 directory).

Specifying a pool using an alternative API

There is an another way of defining subpools as well: make a function for defining each subpool.

def pool_definition_api_with_separate_subpools():
    Create and return a pool by calling up other functions
    for defining the subpools. Also demonstrate customization
    of pool properties and inserting default values from file
    or the command line.
    from parampool.pool.Pool import Pool
    pool = Pool()
    pool.subpool('Main pool')
    pool = motion_pool(pool)
    pool = body_and_envir_pool(pool)
    pool = numerics_pool(pool)
    pool = plot_pool(pool)
    pool.update()  # finalize pool construction

    from parampool.pool.UI import set_data_item_attribute
    # Change default values in the web GUI
    import parampool.pool.DataItem
    parampool.pool.DataItem.DataItem.defaults['minmax'] = [0, 100]
    parampool.pool.DataItem.DataItem.defaults['range_steps'] = 500
    # Can also change 'number_step' for the step in float fields
    # and 'widget_size' for the width of widgets

    # Let all widget sizes be 6, except for Time step
    pool = set_data_item_attribute(pool, 'widget_size', 6)
    pool.get('Time step').data['widget_size'] = 4

    # Example on editing hardcoded defaults in the model files
    # (not necessary, but a possible technique along with
    # setting defaults in the pool, in a file, or on the command line)
    from parampool.pool.UI import set_defaults_in_model_file
    flask_modelfile = ''
    django_modelfile = os.path.join(
        'motion_and_forces_with_pool', 'app', '')
    if os.path.isfile(flask_modelfile):
        set_defaults_in_model_file(flask_modelfile, pool)
    elif os.path.isfile(django_modelfile):
        set_defaults_in_model_file(django_modelfile, pool)

    return pool

def motion_pool(pool, name='Initial motion data'):
        name='Initial velocity', default=5.0, symbol='v_0',
        unit='m/s', help='Initial velocity',
        str2type=float, widget='float',
        validate=lambda data_item, value: value > 0)
        name='Initial angle', default=45, symbol=r'\theta',
        widget='range', minmax=[0,90], str2type=float,
        help='Initial angle',
        validate=lambda data_item, value: 0 < value <= 90)
        name='Spinrate', default=50, symbol=r'\omega',
        widget='float', str2type=float, unit='1/s',
    return pool

def body_and_envir_pool(pool, name='Body and environment data'):
        name='Wind velocity', default=0.0, symbol='w',
        help='Wind velocity in positive x direction.', unit='m/s',
        minmax=[-50, 50], number_step=0.5,
        widget='float', str2type=float)
        name='Mass', default=0.1, symbol='m',
        help='Mass of body.', unit='kg',
        widget='float', str2type=float,
        validate=lambda data_item, value: value > 0)
        name='Radius', default=0.11, symbol='R',
        help='Radius of spherical body.', unit='m',
        widget='float', str2type=float,
        validate=lambda data_item, value: value > 0)
    return pool

def numerics_pool(pool, name='Numerical parameters'):
        name='Method', default='RK4',
        options=['RK4', 'RK2', 'ForwardEuler'],
        help='Numerical solution method.')
        name='Time step', default=None, symbol=r'\Delta t',
        widget='textline', unit='s', str2type=eval,
        help='None: ca 500 steps, otherwise specify float.')
    return pool

def plot_pool(pool, name='Plot parameters'):
        name='Plot simplified motion', default=True,
        help='Plot motion without drag and lift forces.')
        name='New plot', default=True,
        help='Erase all old curves.')
    return pool

This application, found in the directory flask_pool3 (with a corresponding Django counterpart), will now be used to illustrate three important additional features of Parampool:

  1. documentation of the application is in an external file doc.html
  2. the name of a parameter can be a mathematical symbol
  3. parameters can have multiple values for investigating many parameter sets at once

File with documentation of the application

We have in the section More input parameters and results seen that the doc string of the compute function may contain a mathematical description of the problem with rich typesetting (using DocOnce syntax). It is also possible to make such a description in a separate file. Any HTML file will work, and the filename is specified by the doc argument to generate.

LaTeX symbol as parameter name

One can add a mathematical LaTeX symbol for the parameter names (the symbol keyword argument). This symbol can either be displayed as the parameter's complete name, or the symbol can be added to the standard name of the parameter. The choice is set by the latex_name keyword argument in the generate call in

from parampool.generator.flask import generate
from compute import compute_motion_and_forces_with_pool_loop, \

         MathJax=True, doc=open('doc.html', 'r').read(),

The values of latex_name can be 'symbol', meaning symbol only, or 'text, symbol', meaning that the ordinary name is followed by a comma and the symbol.

Multiple input values for parameters

We can specify multiple values for parameters whose input fields are pure text fields. For example, for the wind velocity (\( w \)) parameter we can assign two values separated by the & character: 0 & -8. Calling pool.get_values('Wind velocity') will then return a list [0, -8] rather than one number. We can hence easily make a loop over the multiple values for each parameter where we use pure text as input. Our compute function looks in this case like this:

def compute_motion_and_forces_with_pool_loop(pool):
    html = ''
    initial_angle = pool.get_value('Initial angle')
    method = pool.get_value('Method')
    new_plot = pool.get_value('New plot')  # should be True here
    plot_simplified_motion = pool.get_value('Plot simplified motion')
    for initial_velocity in pool.get_values('Initial velocity'):
        for spinrate in pool.get_values('Spinrate'):
            for m in pool.get_values('Mass'):
                for R in pool.get_values('Radius'):
                    for dt in pool.get_values('Time step'):
                        for w in pool.get_values('Wind velocity'):
                            html += compute_motion_and_forces(
                                initial_velocity, initial_angle,
                                spinrate, w, m, R, method, dt,
                                plot_simplified_motion, new_plot)
    return html

Note that we accumulate the HTML code returned from the compute function compute_motion_and_forces that runs the simulation and returns the results as HTML code. Figure 11 features a mathematical description of the application, LaTeX symbols as parameter names, and two input values for two parameters, leading to \( 2\times 2=4 \) runs, and hence four lines of plots. The example demonstrates how easy it is to quickly perform parameter studies by simply 1) writing the compute function with loops and pool.get_values, 2) separating input values by & in the GUI.

Figure 11: Web interface with documentation, LaTeX symbols, and multiple input values.

However, writing nested loops for a lot of parameters is unnecessary tedious. We can use the itertools.product function to compute all possible combinations and simplify the loop. This function takes a set of lists or tuples and returns an iterator over all combinations of all the elements in the lists/tuples. As an example,

>>> import itertools
>>> values1 = [1, -1]
>>> values2 = [2, 4, 6]
>>> combinations = itertools.product(values1, values2)
>>> for combination in combinations:
...     print combination
(1, 2)
(1, 4)
(1, 6)
(-1, 2)
(-1, 4)
(-1, 6)

The single loop is equivalent to

for value1 in values1:
    for value2 in values2:
        combination = (value1, value2)
	print combination

With an unknown number of lists as arguments to itertools.prodct one can use the construction

values = [values1, values2]
combinations = itertools.product(*values)

With these ideas we can generalize our implementation of nested loops over multiple values of the parameters:

def compute_motion_and_forces_with_pool_loop_product(pool):
    import itertools
    initial_angle = pool.get_value('Initial angle')
    method = pool.get_value('Method')
    new_plot = pool.get_value('New plot')  # should be True here
    plot_simplified_motion = pool.get_value('Plot simplified motion')
    # Make list of all names with potentially multiple values
    names = """
    Initial velocity
    Time step
    Wind velocity
    names = [line.strip() for line in names.splitlines()]
    # Get all values of all parameters
    values = [pool.get_values(name) for name in names]
    combinations = itertools.product(*values)
    html = ''
    for combination in combinations:
        initial_velocity, spinrate, m, R, dt, w = combination
        html += compute_motion_and_forces(
            initial_velocity, initial_angle,
            spinrate, w, m, R, method, dt,
            plot_simplified_motion, new_plot)
    return html

This is the type of implementation recommended in applications.