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Structure#

Brightway Components#

Brightway is split into several main packages:

  • Brightway25 is the umbrella package, as well as documentation. When installing this package, it will pull as dependencies the following packages:

    • Brightway2-data handles storing and searching all data sources (databases, LCIA methods, etc.).

    • Brightway2-calc does LCA calculations.

    • Brightway2-io tools for the import, export, and management of inventory databases and impact assessment methods. LCA calculations.

    • Brightway2-analyzer analyzes input data like databases and methods, as well as the result of LCA calculations.

    • Brightway2-parameters Library for storing, validating, and calculating with parameters.

Projects#

Data in Brightway2 is structured in a hierarchy. At the top level, we have projects. A project is self-contained, with its own copy of data, LCIA methods, calculations, assumptions, and any other data you need. Each project is completely independent of other projects.Projects are saved as subdirectories in the file system.

image

Inside a project we have a number of objects that store data. The most common data objects are inventory databases and impact assessment methods. However, non-LCA data can also be included. For example, a set of vehicle registrations and lifetimes could also be stored in a project, and used to generate fleet-based scenarios for sustainability assessment of mobility services.

Project are created in a suitable location for your operating system with the help of the appdirs library.

Projects can be easily created, copied, manipulated, or deleted. See the projects example notebook.

Warning

Brightway2 uses atomic file writes to prevent data corruption, but files are hard; you should make regular backups using the backup-data-directory function.

Inventory Databases#

In Brightway2, a database is the object used to organize a set of nodes and edges in a life cycle inventory graph of the industrial supply chain and natural world. For example, a specific version of ecoinvent could be a database, but so would a set of biosphere flows, as biosphere flows are also nodes in our inventory graph. Databases can be big, like ecoinvent, or as small as a single dataset. You can have as many databases as you like, and databases can have links into other databases. You can also have databases that each depend on each other.

SimaPro differentiates between what it calls projects and libraries, but both would be a database in Brightway2.

Databases can be easily created, copied, modified, iterated over, searched, and delted. See the databases example notebook.

Activities and Exchanges#

In the database, nodes are called activities, and include transforming and market activities, but also products and biosphere flows. Edges are called exchanges, and describe a link between two nodes. An exchange could describe the input of a product to a transforming activity, or an emission of a biosphere flow by an activity, or the name and amount of a product produced by an activity.

Activity data format#

A database consists of inventory datasets, and inventory datasets are text documents, human-readable data that you can manipulate manually in a text editor, or change en masse programmatically. Because they can be exported as text, and in a format that is accessible to almost every computer language (JSON), activity datasets can be easily exported and used by other programs.

Inventory datasets have a very flexible and free text form; even an empty dictionary (e.g. {}) is a valid LCI dataset in Brightway2. However, some fields are suggested for common use. Note that you can always add extra fields as needed by your application. Here is a selection from an example dataset from the US LCI:

{
 'categories': ['Wood Product Manufacturing', 'Softwood Veneer and Plywood Mnf.'],
 'location': 'RNA',
 'name': 'Green veneer, at plywood plant, US PNW',
 'type': 'process',
 'unit': 'kilogram'}
 'exchanges': [{
   'amount': 1.0,
   'code': 6,
   'group': 2,
   'input': ('US LCI', '6ddb4cc00f9e42aa48515248256c31dc'),
   'type': 'production',
   'uncertainty type': 0},
  {'amount': 7.349999999999999e-06,
   'code': 5,
   'group': 4,
   'input': ('biosphere', '51447e58e03a40a2bbd9abf45214b7d3'),
   'type': 'biosphere',
   'uncertainty type': 0}],
}

The document structure is:

  • name (string): Name of this activity.

  • type (string): If this is "process", or omitted completely, Brightway2 will treat this as a inventory process with inputs and output(s). If you want to store additional information in a Database outside of the list of processes, specify a custom type here. For example, the list of biosphere flows is also an inventory database, but as these are flows, not processes, they have the type "emission". Similarly, if you wanted to separate processes and products, you could create database entries for the products, with the type "product".

  • categories (list of strings, optional): A list of categories and subcategories. No length limits.

  • location (string, optional): A location identifier. Default is GLO, but this can be changed in the user-preferences.

  • unit (string): Unit of this activity. Units are normalized when written to disk.

  • exchanges (list): A list of activity inputs and outputs, with its own schema.

    • input (database name, database code): The technological activity that is linked to, e.g. ("my new database", "production of ice cream") or ('biosphere', '51447e58e03a40a2bbd9abf45214b7d3'). See also dataset-codes.

    • type (string): One of production, technosphere, and biosphere. See exchanges.

    • amount (float): Amount of this exchange.

    • uncertainty type (integer): Integer code for uncertainty distribution of this exchange, see uncertainty-type for more information. There can be other uncertainty fields as well.

    • comment (string, optional): A comment on this exchange. Used to store pedigree matrix data in ecoinvent v2.


Uniquely identifying activities#

Linking activity datasets within and between databases requires a way to uniquely identify each dataset - Brightway2 calls this unique identifier a code. A code can be a number, like 1, or a string of numbers and letters, like swiss ch33se. When you create datasets manually, you will need to assign each dataset a code. When you import a database, the codes will be automatically generated for you.

Activity hashes#

When you import an ecospold or SimaPro dataset, the data format does not provide a way to uniquely identify each dataset. Brightway2 will generate codes that look like a bunch of nonsense, e.g.: 6d336c64e3a0ff08dee166a1dfdf0946. In this case, Brightway2 identifies an activity or flow with the MD5 hash of a few attributes: For ecoinvent 2, the name, location, unit, and categories. For ecoinvent 3, the activity and reference product names.

Activities must be uniquely identified#

Activities are identified by their database name and a unique code. A code is a string of letters and numbers that uniquely identifies an activity within the database. Codes can be written by humans, e.g. "Chris's first pony", or generated by by the computer using an algorithm.

Activities do not have very many required fields; aside from database and code, the only other required field is name, but most activities will have a location and unit as well. If no type is specified for an activity, then the activity is assumed to be a process. Other types include product and biosphere for biosphere flows. Activity type is used to determine whether an activity should be placed in the biosphere or technosphere matrices during LCA calculations.

Exchanges are links between two activities of any type. Exchanges have an input and an output: input is the activity being consumed or produced, and output is the consumer or producer. Exchanges should also have an amount and a type. Common types include technosphere, biosphere, and production. Multiple exchanges between two activities are allowed, and will be added together during LCA calculations.

Many activities have a reference product, which is an exchange of type production where the input is the same as the output.

Brightway2 allows multioutput processes; you are responsible for making sure the final system make mathematical sense (see multioutput processes in LCA).

Exchange data format#

Exchanges are a list of the inputs and outputs of an activity. For example an activity might consume some resources, emit some emissions, and have other technological goods as emissions. Each activity also has at least one technological output.

Each exchange has a type. There are three standard exchange types in Brightway2, but you can define your own if you need to define different kinds of systems.

Production exchanges#

A production exchange defines how much of the output is produced by an activity. For example, the process “make a fizzbang” would produce one kilogram of fizzbang (the amount is normally one, but doesn’t have to be).

Production exchanges have the type production.

Note

A production exchange is not required. A default value of one will be applied if no production exchange is defined. This default value is usually the most logical amount, so should only be changed in special circumstances.

Warning

Using a production value other than one can be confusing. See the blog post What happens with a non-unitary production amount in LCA?.

Warning

Multioutput processes (i.e. more than one production process) can be used in Brightway2, but only under special circumstances. See the blog post Multi-output processes in matrix-based LCA.

Substitution exchanges#

A substitution exchange is used in multi-output processes to indicate the avoided production of a product by another activity. Substitution exchanges have positive values, and the type [substitution].

Technosphere exchanges#

A technosphere exchange is a process input from the technosphere, i.e. the industrial economy. For example, the process “make a fizzbang” could have an input of seven kilograms of lollies.

Technosphere exchanges have the type technosphere.

Biosphere exchanges#

A biosphere exchange is a consumption of a resource or and emission to the environment associated with a process; its value will be placed in the biosphere matrix.

Biosphere exchanges have the type biosphere.

Database is a subclass of DataStore#

Much of the functionality of Database objects is provided by its parent class, datastore. The normal methods provided by a data store are:

  • write(data): Write data to disk

  • load: Load data from disk

  • register: Register object with metadata store

  • deregister: Remove object from metadata store

  • copy(name): Create a new object with name name

  • backup: Write backup of data

  • validate(data): Validate data using this object’s validator

Data store objects are instantiated with the object name, e.g. DataStore("name goes here").

Brightway2-data defines the following data stores:

  • SingleFileDatabase <single-file-database>

  • JSONDatabase <json-database>

  • method

  • weighting

  • normalization

The schema for an LCI dataset in voluptuous is:

{
    Optional("categories"): Any(list, tuple),
    Optional("location"): object,
    Optional("unit"): basestring,
    Optional("name"): basestring,
    Optional("type"): basestring,
    Optional("exchanges"): [exchange]
}

Where an exchange is:

{
    Required("input"): valid_tuple,
    Required("type"): basestring,
    Required("amount"): Any(float, int),
    Optional("uncertainty type"): int,
    Optional("loc"): Any(float, int),
    Optional("scale"): Any(float, int),
    Optional("shape"): Any(float, int),
    Optional("minimum"): Any(float, int),
    Optional("maximum"): Any(float, int)
}

Note

Database documents can be validated with bw2data.validate.db_validator(my_data), or Database("my database name").validate(my_data).

Getting the signs right#

Brightway uses the following rules to set values in the technosphere and biosphere matrices:

  • [biosphere] exchange values are inserted into the biosphere matrix without any modification.

  • [production] and [substitution] exchanges are inserted into the technosphere matrix without any modification.

  • [technosphere] exchanges values are multiplied by negative one, and then inserted into the technosphere matrix.

In the technosphere matrix, negative values represent the consumption of products, while positive values represent the production of products. Substitution exchanges are positive because this forces the substituted activity to have a negative production amount, representing the avoided production pathway.

These rules are consistent with and grow out of the traditional Leontief inverse of IO tables \(x = (I - A)^{-1}d\).

As a consequence of these rules, a technosphere exchange with a negative value is the same as a production exchange, and vice-versa.

Biosphere exchange amounts can occasionally be negative, and some characterization factors are also negative. The default metadata in Brightway follows ecoinvent system assumptions about biosphere flow categories:

  • Biosphere flows whose categories are [air], [soil], and [water] are emissions into the natural environment.

  • Biosphere flows with the category [natural resource] are consumption of natural resources from the natural environment.

Biosphere exchanges with negative values reverse these assumption; so, a biosphere flow of -2 kg of carbon dioxide with the category air would be the removal of carbon dioxide from the natural environment. The signs of biosphere exchanges don’t really matter, but they should be consistent with the signs of your impact assessment characterization factors. See also the notebook on negative Biosphere flows and CFs.

Databases can be stored in different ways#

The default storage backend for databases stores each database in a separate file. This is the easiest and most convenient approach for most cases. However, Brightway2 also supports pluggable database backends, which can change how databases are stored and queried.

Brightway2-data also provides bw2data.backends.JSONDatabase, which stores each dataset as a separate file serialized to JSON. This approach works well with version-control systems, as each dataset change can be saved individually. Use of JSONDatabase is shown in a simple ipython notebook.

Before using JSONDatabase, please read its technical documentation carefully: json-database. To create a JSONDatabase, use Database("my db name", backend="json"). To switch backends for a database, use convert_backend <switching-backends>.

custom-backends, such as using an actual relational database, can also be defined.

Biosphere database#

When you run bw2setup() in a python shell, Brightway2 will install a special biosphere3 database. This database has all the resource and emission flows from the ecoinvent database, version 2.

You can define biosphere flows - resources and emissions - in any database you like, but it is probably best to use the pre-defined flows in the biosphere database whenever you can. If you need to add some custom flows, feel free to create a separate database.

You can also change the name for the default biosphere database in the user preferences <user-preferences>.

Impact Assessment#

In Brightway2, each impact assessment method is a set of characterization factors for a set of biosphere flows. Each impact category and subcategory is a separate method, and each method is stored and calculated separately.

Methods are identified by a list of names, which could be as simple as:

("I scream", "you scream", "we all scream", "for ice cream")

which is probably most applicable for those who are particularly concerned with ice cream resource depletion; a more typical example is:

('ecological scarcity 1997', 'total', 'total')

Impact assessment method names can have any length and number of qualifiers - there is nothing special or sacred about three levels - but must always be a list of strings.

Warning

For technical reasons, impact assessment names must be stored as a tuple, not a list, i.e. they must have () at the beginning and end, and not [].

Method metadata#

Method metadata is a normal dictionary, and is indexed in the methods object. The object methods is a special dictionary that saves itself whenever values change, but is otherwise still a normal dictionary. new_method.metadata is an alias for methods. So, to change the metadata, do:

methods[('foo',)] = {'bar': True, ...}

Or to chance a single value:

methods[('IPCC 2007', 'climate change', 'GWP 100a')]['timeframe'] = 100

Note that after changing a single value, you will need to flush the changes to disk:

methods.flush()

Methods should have the following metadata:

  • description: A description of this method or submethod.

  • unit: The unit of this method or submethod.

In addition, the metadata abbreviation is generated automatically.

LCIA method documents#

The impact assessment method documents are quite simple - indeed, it is a bit of a stretch to call them documents at all. Instead, they are a list of biosphere flow references, characterization factors, and locations. All LCIA methods in Brightway2 are regionalized, though the default installed methods only provide global characterization factors. Here is a simple example:

from brightway2 import *
Method(('ecological scarcity 1997', 'total', 'total')).load()[:5]

This returns the following:

[[(u'biosphere', u'21c70338ff2e1cdc8e468f4c90f113a1'), 32000, u'GLO'],
 [(u'biosphere', u'86a37cf9e44593f1c41fdce53de27715'), 32000, u'GLO'],
 [(u'biosphere', u'a8cc9c61aa343fa01532bb16cec7f90d'), 32000, u'GLO'],
 [(u'biosphere', u'b0a29177e77471a49b5a7d6a88212bf8'), 32000, u'GLO'],
 [(u'biosphere', u'72c1cf2fee31a2cb6cdc39abda29a0df'), 32000, u'GLO']]

Each list elements has two required components and a third optional component.

  1. A reference to a biosphere flow, e.g. (u'biosphere', u'21c70338ff2e1cdc8e468f4c90f113a1').

  2. The numeric characterization factor. This can either be a number, or a uncertainty dictionary (see uncertainty-type).

  3. An optional location, used for regionalized impact assessment. The global location GLO is inserted as a default if not location is specified.

Note

LCIA method documents can be validated with bw2data.validate.ia_validator(my_data), or Method(("my", "method", "name")).validate(my_data).

Default LCIA methods#

When you run bw2setup(), Brightway2 will install around 700 default LCIA methods, as provided by the ecoinvent center. These LCIA methods will work for both ecoinvent 2 and 3.

Parameterized datasets#

Brightway2 supports variables and formulas stored as strings, similar to other LCA software. So instead of defining a fixed value for an exchange, you could enter a formula of “fuel_efficiency * average_distance”, where both “fuel_efficiency” and “average_distance” were variables stored in a special way, and maybe even parameterized themselves. Parsing strings is not trivial, and so the machinery to handle such parameterization is a bit complex:

image

Warning

Parameterized inventory datasets only work with databases that use the default SQLite backend.

Groups#

Parameters are tricky because you have to parse and understand dependencies in formula strings - where if “efficiency” defined, and is it a Python reserved term or a function or a variable, etc. To make these dependencies explicit, Brightway2 uses the ideas of groups to collect parameters, just like databases collect inventory datasets. Each parameter belongs to a group, and inside a group each parameter has to have a unique name. Groups also have unique names, and are defined at the three different levels that parameters exist: project, database, or a set of activities. Groups cannot cross levels.

Groups form a hierarchy used to evaluate and find symbols, with project parameters at the top, and activity parameters at the bottom. When parsing an activity parameter formula, unknown variable names will be searched in that activity parameter set of variables, then in the database parameters defined for the database the activity is in, and finally in the project parameters. An missing value will be taken as soon as it is found - so if “efficiency” exists in a database parameter group and the project parameter group, its value will be taken from the database parameters.

Note the following restrictions on groups:

  • The group name ‘project’ is reserved for the group of project parameters

  • Database names are reserved for database parameters (it is also their group name)

  • Activity parameter groups can include more than one activity, but cannot span multiple databases

  • Single activities cannot be in multiple groups

  • Group dependencies cannot be circular

These restrictions are enforced in the database, so you can’t screw up your data, but they might explain any errors you encounter.

Active versus passive parameters#

Some background datasets have lots of parameters, and one doesn’t necessarily want them all to be imported into the Brightway parameter machinery - after all, they have been resolved already. We therefore use a distinction between active and passive parameters. Active parameters are stored in a special SQLite database for parameters, and their formulas are parsed and checked to make sure there are no missing or unknown symbols. Active parameters are recalculated whenever their upstream groups change, and can be used in dynamic calculation. Passive parameters are stored in either Database instances (as the key parameters in the metadata), Activity objects (as the key parameters in the metadata), or in Exchanges (as the key formula in the exchange data). They are not evaluated or otherwise used.

The parameters manager has functions for activating activities and exchanges.

Parameters manager#

The most common way to interact with parameters data is through the parameters manager, provided as parameters.

Peewee objects#

At a finer level of control, the parameterized table objects use peewee objects directly, so you will use some different syntax than with [Activity] and [Exchange] (see the parameters source code). The long-term goal is to transition all objects to peewee directly, instead of using proxies.

The parameters framework is centered around the Group, ProjectParameter, DatabaseParameter, and ActivityParameter classes <parameters>.

Here are some examples of peewee-style queries:

Group.create(name="some name")

group, created = Group.get_or_create(name="some name")

for obj in DatabaseParameter.select().where(
    DatabaseParameter.database="some db"):
  print(obj.name, obj.amount, obj.formula)

ActivityParameter.update(amount = some_new_value
    ).where(ActivityParameter.name="some name").execute()

ProjectParameter.delete().where(ProjectParameter.name="some name"
    ).execute()

Intermediate and processed data#

Both inventory datasets and impact assessment methods are stored as structured text files, stored in the intermediate folder. These files are not efficient when constructing the technosphere, biosphere, and characterization matrices. Brightway2 also has a processed folder, which stores only the data needed to construct the various computational matrices. These data are stored as numpy structured arrays.

For both databases and LCIA methods, the method .write(some_data) will write an intermediate data file, while the subsequent method .process() will transform the intermediate data file to an array. All extraneous information is removed, and only the numeric values needed are retained. Put another way, processing transforms unstructured data documents to a highly-structured binary form for calculations. write and process are intentionally separate, as it is sometimes desirable to do one and not the other.

building-matrices describes how processed data are turned into matrices for LCA calculations.

Warning

Every time you save a new version of an inventory database or an impact assessment method, e.g. with my_database.write(my_data), be sure to also call my_database.process(), or your changes will not be used in LCA calculations.

Processing data#

Processing data converts document data to a binary form tailored for creating matrices (a NumPy array).

Mappings#

Some LCA data is not numerical, like locations and dataset codes. We need numerical representations of these values to construct the processed data arrays, however. In this case, we create a special dictionary that maps each unique data value to an integer index. Brightway2 uses two such mappings:

  • mapping <mapping>: Maps inventory objects (activities, biosphere flows, and anything else that would appear in a supply chain graph) to indices.

  • geomapping: Map locations (both inventory and regionalized impact assessment) to indices.

Items are added to mappings using .add(keys), and removed using .delete(keys). However, managing the different mappings is done for you automatically.

Cataloging what we have - Metadata stores#

The building blocks in Brightway2 are LCI databases, LCIA methods, etc. However, we also need to keep track of which LCI databases and LCIA methods we have, as well as some additional information about them. For example, LCIA methods have units, and databases can have version numbers. A metadata store stores information about data objects like databases and methods.

The base class for metadata is serialized-dict, which is basically a normal Python dictionary that can be easily saved or loaded (i.e. serialized) to or from a JSON file. These files can be easily edited in a normal text editor.

Brightway2 defines the following metadata stores:

  • databases: LCI databases

  • methods: LCIA methods (characterization factors)

  • normalizations: LCIA normalization factors

  • weightings: LCIA weighting factors

Metadata should be singletons#

There should be only one instance of each metadata store, to avoid having conflicting data (the singleton pattern). The normal pattern is to instantiate each class in the same file as the class pattern:

class MyObjects(bw2data.serialization.SerializedDict):
    file = "sweet-peppers.json"

myobjects = MyObjects()

Using metadata stores#

Metadata stores are mostly useful when examining which objects are available:

for name in databases:
   print name
"a database name" in databases

Metadata stores are also used when deleting data objects:

del databases["some database to delete"]

Finally, and hopefully not surpisingly, metadata stores can be used to get the actual data object metadata:

methods[methods.random()]
>> {u'abbreviation': u'recipe-endpoint-ha-wo-lthc.0ba25d5fd76e35b3125224ce78d37151',
    u'unit': u'points'}