# Basic Usage¶

## Importing a model and getting started¶

To begin, we must first load the PySD module, and use it to import a supported model file:

```
import pysd
model = pysd.read_vensim('Teacup.mdl')
```

This code creates an instance of the PySD class loaded with an example model that we will use as the system dynamics equivalent of ‘Hello World’: a cup of tea cooling to room temperature.

To view a synopsis of the model equations and documentation, call the `doc()`

method of the model class. This will generate a listing of all the model elements, their documentation, units, equations, and initial values, where appropriate. Here is a sample from the teacup model:

```
>>> print model.doc()
```

Note

You can also load an already translated model file, what will be faster as you will load a Python file:

```
import pysd
model = pysd.load('Teacup.py')
```

Note

The functions `read_vensim()`

, `read_xmile()`

and `load()`

have optional arguments for advanced usage, you can check the full description in User Functions Reference or using `help()`

e.g.:

```
import pysd
help(pysd.load)
```

## Running the Model¶

The simplest way to simulate the model is to use the `run()`

command with no options. This runs the model with the default parameters supplied by the model file, and returns a Pandas dataframe of the values of the stocks at every timestamp:

```
>>> stocks = model.run()
t teacup_temperature
0.000 180.000000
0.125 178.633556
0.250 177.284091
0.375 175.951387
…
```

Pandas gives us simple plotting capability, so we can see how the cup of tea behaves:

```
stocks.plot()
plt.ylabel('Degrees F')
plt.xlabel('Minutes')
```

To show a progressbar during the model integration the progress flag can be passed to the `run()`

command, progressbar package is needed:

```
>>> stocks = model.run(progress=True)
```

## Outputting various run information¶

The `run()`

command has a few options that make it more useful. In many situations we want to access components of the model other than merely the stocks – we can specify which components of the model should be included in the returned dataframe by including them in a list that we pass to the `run()`

command, using the return_columns keyword argument:

```
>>> model.run(return_columns=['Teacup Temperature', 'Room Temperature'])
t Teacup Temperature Room Temperature
0.000 180.000000 75.0
0.125 178.633556 75.0
0.250 177.284091 75.0
0.375 175.951387 75.0
…
```

If the measured data that we are comparing with our model comes in at irregular timestamps, we may want to sample the model at timestamps to match. The `run()`

function gives us this ability with the return_timestamps keyword argument:

```
>>> model.run(return_timestamps=[0,1,3,7,9.5,13.178,21,25,30])
t Teacup Temperature
0.0 180.000000
1.0 169.532119
3.0 151.490002
7.0 124.624385
9.5 112.541515
…
```

## Retrieving totally flat dataframe¶

The subscripted variables, in general, will be returned as *xarray.DataArray*s in the output *pandas.DataFrame*. To get a totally flat dataframe, like Vensim outuput the flatten=True when calling the run function:

```
>>> model.run(flatten=True)
```

## Setting parameter values¶

In many cases, we want to modify the parameters of the model to investigate its behavior under different assumptions. There are several ways to do this in PySD, but the `run()`

function gives us a convenient method in the params keyword argument.

This argument expects a dictionary whose keys correspond to the components of the model. The associated values can either be a constant, or a Pandas series whose indices are timestamps and whose values are the values that the model component should take on at the corresponding time. For instance, in our model we can set the room temperature to a constant value:

```
model.run(params={'Room Temperature': 20})
```

Alternately, if we believe the room temperature is changing over the course of the simulation, we can give the run function a set of time-series values in the form of a Pandas series, and PySD will linearly interpolate between the given values in the course of its integration:

```
import pandas as pd
temp = pd.Series(index=range(30), data=range(20, 80, 2))
model.run(params={'Room Temperature':temp})
```

If the parameter value to change is a subscripted variable (vector, matrix…), there are three different options to set new value. Suposse we have ‘Subscripted var’ with dims `['dim1', 'dim2']`

and coordinates `{'dim1': [1, 2], 'dim2': [1, 2]}`

. A constant value can be used and all the values will be replaced:

```
model.run(params={'Subscripted var': 0})
```

A partial *xarray.DataArray* can be used, for example a new variable with ‘dim2’ but not ‘dim2’, the result will be repeated in the remaining dimensions:

```
import xarray as xr
new_value = xr.DataArray([1,5], {'dim2': [1, 2]}, ['dim2'])
model.run(params={'Subscripted var': new_value})
```

Same dimensions *xarray.DataArray* can be used (recommended):

```
import xarray as xr
new_value = xr.DataArray([[1,5],[3,4]], {'dim1': [1, 2], 'dim2': [1, 2]}, ['dim1', 'dim2'])
model.run(params={'Subscripted var': new_value})
```

In the same way, a Pandas series can be used with constan values, partially defined *xarray.DataArrays* or same dimensions *xarray.DataArrays*.

Note

That once parameters are set by the run command, they are permanently changed within the model. We can also change model parameters without running the model, using PySD’s `set_components(params={})`

method, which takes the same params dictionary as the run function. We might choose to do this in situations where we’ll be running the model many times, and only want to spend time setting the parameters once.

Note

If you need to know the dimensions of a variable, you can check them by using `get_coords(variable__name)`

function:

```
>>> model.get_coords('Room Temperature')
None
>>> model.get_coords('Subscripted var')
({'dim1': [1, 2], 'dim2': [1, 2]}, ['dim1', 'dim2'])
```

this will return the coords dictionary and the dimensions list if the variable is subscripted or ‘None’ if the variable is an scalar.

Note

If you change the value of a lookup function by a constant, the constant value will be used always. If a *pandas.Series* is given the index and values will be used for interpolation when the function is called in the model, keeping the arguments that are included in the model file.

If you change the value of any other variable type by a constant, the constant value will be used always. If a *pandas.Series* is given the index and values will be used for interpolation when the function is called in the model, using the time as argument.

If you need to know if a variable takes arguments, i.e., if it is a lookup variable, you can check it by using `get_args(variable__name)`

function:

```
>>> model.get_args('Room Temperature')
[]
>>> model.get_args('Growth lookup')
['x']
```

## Setting simulation initial conditions¶

Finally, we can set the initial conditions of our model in several ways. So far, we’ve been using the default value for the initial_condition keyword argument, which is ‘original’. This value runs the model from the initial conditions that were specified originally by the model file. We can alternately specify a tuple containing the start time and a dictionary of values for the system’s stocks. Here we start the model with the tea at just above freezing:

```
model.run(initial_condition=(0, {'Teacup Temperature': 33}))
```

The new value setted can be a *xarray.DataArray* as it is explained in the previous section.

Additionally we can run the model forward from its current position, by passing the initial_condition argument the keyword ‘current’. After having run the model from time zero to thirty, we can ask the model to continue running forward for another chunk of time:

```
model.run(initial_condition='current',
return_timestamps=range(31,45))
```

The integration picks up at the last value returned in the previous run condition, and returns values at the requested timestamps.

There are times when we may choose to overwrite a stock with a constant value (ie, for testing). To do this, we just use the params value, as before. Be careful not to use ‘params’ when you really mean to be setting the initial condition!

## Querying current values¶

We can easily access the current value of a model component using curly brackets. For instance, to find the temperature of the teacup, we simply call:

```
model['Teacup Temperature']
```

If you try to get the current values of a lookup variable the previous method will fail as lookup variables take arguments. However, it is possible to get the full series of a lookup or data object with `get_series_data()`

method:

```
model.get_series_data('Growth lookup')
```

## Supported functions¶

Vensim functions include:

Vensim | Python Translation |
---|---|

= | == |

< | < |

> | > |

>= | >= |

<= | <= |

^ | ** |

ABS | np.abs |

MIN | np.minimum |

MAX | np.maximum |

SQRT | np.sqrt |

EXP | np.exp |

LN | np.log |

PI | np.pi |

SIN | np.sin |

COS | np.cos |

TAN | np.tan |

ARCSIN | np.arcsin |

ARCCOS | np.arccos |

ARCTAN | np.arctan |

ELMCOUNT | len |

INTEGER | functions.integer |

QUANTUM | functions.quantum |

MODULO | functions.modulo |

IF THEN ELSE | functions.if_then_else |

PULSE TRAIN | functions.pulse_train |

RAMP | functions.ramp |

INVERT MATRIX | functions.invert_matrix |

VMIN | functions.vmin |

VMAX | functions.vmax |

SUM | functions.sum |

PROD | functions.prod |

LOGNORMAL | np.random.lognormal |

STEP | functions.step |

PULSE | functions.pulse |

EXPRND | np.random.exponential |

POISSON | np.random.poisson |

RANDOM NORMAL | functions.bounded_normal |

RANDOM UNIFORM | np.random.rand |

DELAY1 | functions.Delay |

DELAY3 | functions.Delay |

DELAY N | functions.DelayN |

DELAY FIXED | functions.DelayFixed |

FORECAST | functions.Forecast |

SAMPLE IF TRUE | functions.SampleIfTrue |

SMOOTH3 | functions.Smooth |

SMOOTH N | functions.Smooth |

SMOOTH | functions.Smooth |

INITIAL | functions.Initial |

XIDZ | functions.XIDZ |

ZIDZ | functions.XIDZ |

GET XLS DATA | external.ExtData |

GET DIRECT DATA | external.ExtData |

GET XLS LOOKUPS | external.ExtLookup |

GET DIRECT LOOKUPS | external.ExtLookup |

GET XLS CONSTANTS | external.ExtConstant |

GET DIRECT CONSTANTS | external.ExtConstant |

GET XLS SUBSCRIPT | external.ExtSubscript |

GET DIRECT SUBSCRIPT | external.ExtSubscript |

np corresponds to the numpy package