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Storing Custom Data

Introduction

It is often very convenient to store any arbitrary data inside an Entity. A few examples are:

  • calculated properties of atoms
  • sequence conservation of a residue
  • interaction energy of a substructure with its surrounding
  • fit of a fragment inside an electron density map

In OpenStructure this is supported by the use of generic properties. Most building blocks are derived from GenericPropertyContainer, meaning that arbitrary key-value pairs can be stored in them. In essence, the following classes support generic properties:

The view variants will reflect the generic properties of the handle variants.

A generic property key is always a string, and a value can be one of string, float, int or bool. For each of these data types, methods to retrieve and store values are available both in Python and C++.

Storing and Accessing Data

All OpenStructure building blocks that are GenericPropContainers, have four different methods to store generic data, depending on the data type (i.e. string, float, int or bool).

To store a float value with the key ‘myfloatprop’ in all atoms of an entity:

import math
for atom in entity.GetAtomList():
  val=5*math.sin(0.4*atom.GetPos().x)
  atom.SetFloatProp("myfloatprop", val)

If a GenericProp at a given level (i.e. atom, bond, residue, chain or entity) already exists, it will be overwritten. To check if it exists, use:

exists=atom.HasProp("myfloatprop")
print exists

To access the value of a generic property, we first check if the property exists and then access it, using the method suitable for the data type of the property. For the previously set property myfloatprop of the data type real, at the atom level:

for atom in entity.GetAtomList():
  if atom.HasProp("myfloatprop"):
    print atom.GetFloatProp("myfloatprop")

When trying to access a property that has not been set, or one that has been set, but at a different level, an error is thrown. The same is true when trying to access a property of a different data type, e.g.:

# all of the following lines will throw errors
# error because the property does not exist
print atom.GetFloatProp("unknownprop")

# error because the property was set at another level
print entity.GetFloatProp("myfloatprop")

# error because the data type of the property is different
print atom.GetStringProp("myfloatprop")

Use of Generic Properties in Queries

The Queries can also be used for numeric generic properties (i.e. bool, int, float), but the syntax is slightly different. To access any generic properties, it needs to be specified that they are generic and at which level (chain, residue, atom) they are defined. Therefore, all generic properties start with a ‘g’, followed by an ‘a’, ‘r’ or ‘c’ for atom, residue or chain level respectively. For more details see Queries.

API documentation

class GenericPropertyContainer
HasProp(key)

checks existence of property. Returns true, if the the class contains a property with the given name, false if not.

GetPropAsString(key)

Returns the string representation of a property, or the empty String if the property addressed by key does not exist. Note that this is not the same as trying to get a generic float/int/bool property as a string type; the latter will result in a boost:get exception. Use this method to obtain a representation suitable for output.

GetStringProp(key)
GetStringProp(key, default_value)

Get string property. The first signature raises a GenericPropError error if the property does not exist, the second returns the default value.

GetFloatProp(key)
GetFloatProp(key, default_value)

Get float property. The first signature raises a GenericPropError error if the property does not exist, the second returns the default value.

GetIntProp(key)
GetIntProp(key, default_value)

Get int property. The first signature raises a GenericPropError error if the property does not exist, the second returns the default value.

GetBoolProp(key)
GetBoolProp(key, default_value)

Get bool property. The first signature raises a GenericPropError error if the property does not exist, the second returns the default value.

ClearProps()

Remove all generic properties

SetStringProp(key, value)

Set string property, overriding an existing property with the same name

SetFloatProp(key, value)

Set float property, overriding an existing property with the same name

SetIntProp(key, value)

Set int property, overriding an existing property with the same name

SetBoolProp(key, value)

Set bool property, overriding a property with the same name

RemoveProp(key)

Removes the property with given key, regardless of its type. If the property does not exist, the method has no effect.

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