# Usage¶

Note

This chapter assumes knowledge of the basic concept and difference between work functions and work chains is known and when one should use on or the other.

A workflow in AiiDA is a process (see the process section for details) that calls other workflows and calculations and optionally returns data and as such can encode the logic of a typical scientific workflow. Currently, there are two ways of implementing a workflow process:

This section will provide detailed information and best practices on how to implement these two workflow types.

## Work functions¶

The concept of work functions and the basic rules of implementation are documented in detail elsewhere:

Since work functions are a sub type of process functions, just like calculation functions, their implementation rules are as good as identical. However, their intended aim and heuristics are very different. Where calculation functions are ‘calculation’-like processes that create new data, work functions behave like ‘workflow’-like processes and can only return data. What this entails in terms of intended usage and limitations for work functions is the scope of this section.

### Returning data¶

It has been said many times before: work functions, like all ‘workflow’-like processes, return data, but what does return mean exactly? In this context, the term ‘return’ is not intended to refer to a piece of python code returning a value. Instead it refers to a workflow process recording a data node as one of its outputs, that it itself did not create, but which rather was created by some other process, that was called by the workflow. The calculation process was responsable for creating the data node and the workflow is merely returning it as one of its outputs.

This is then exactly what the workfunction function does. It takes one or more data nodes as inputs, calls other processes to which it passes those inputs and optionally returns some or all of the outputs created by the calculation processes it called. As explained in the technical section, outputs are recorded as ‘returned’ nodes simply by returning the nodes from the function. The engine will inspect the return value from the function and attach the output nodes to the node that represents the work function. To verify that the output nodes are in fact not ‘created’, the engine will check that the nodes are stored. Therefore, it is very important that you do not store the nodes you create yourself, or the engine will raise an exception, as shown in the following example:

# -*- coding: utf-8 -*-
from aiida.engine import workfunction
from aiida.orm import Int

@workfunction
def illegal_workfunction(x, y):
return Int(x + y)

result = illegal_workfunction(Int(1), Int(2))


Because the returned node is a newly created node and not stored, the engine will raise the following exception:

ValueError: Workflow<illegal_workfunction> tried returning an unstored Data node.
This likely means new Data is being created inside the workflow.
In order to preserve data provenance, use a calcfunction to create this node and return its output from the workflow


Note that you could of course circumvent this check by calling store yourself on the node, but that misses the point. The problem with using a workfunction to ‘create’ new data, is that the provenance is lost. To illustrate this problem, let’s go back to the simple problem of implementing a workflow to add two integer and multiply the result with a third. The correct implementation has a resulting provenance graph that clearly captures the addition and the multiplication as separate calculation nodes, as shown in Fig. 9. To illustrate what would happen if one does does not call calculation functions to perform the computations, but instead directly perform them in the work function itself and return the result, consider the following example:

# -*- coding: utf-8 -*-
from aiida.engine import calcfunction, workfunction
from aiida.orm import Int

@workfunction
sum = Int(x + y)
product = Int(sum * z)
return product.store()



Warning

For the documentation skimmers: this is an explicit example on how not to use work functions. The correct implementation calls calculation functions to perform the computation

Note that in this example implementation we explicitly had to call store on the result before returning it to avoid the exception thrown by the engine. The resulting provenance would look like the following:

Fig. 15 The provenance generated by the incorrect work function implementation. Note how the addition and multiplication are not explicitly represented, but are implicitly hidden inside the workflow node. Moreover, the result node does not have a ‘create’ link, because a work function cannot create new data.

However, looking at the generated provenance shows exactly why we shouldn’t. This faulty implementation loses provenance as it has no explicit representations of the addition and the multiplication and the result node does not have a create link, which means that if only the data provenance is followed, it is as if it appears out of thin air! Compare this to the provenance graph of Fig. 9, which was generated by a solution that correctly uses calculation functions to perform the computations. In this trivial example, one may think that this loss of information is not so important, because it is implicitly captured by the workflow node. But a halfway solution may make the problem more apparent, as demonstrated by the following snippet where the addition is properly done by calling a calculation function, but the final product is still performed by the work function itself:

# -*- coding: utf-8 -*-
from aiida.engine import calcfunction, workfunction
from aiida.orm import Int

@calcfunction
return Int(x + y)

@workfunction
product = Int(sum * z)
return product.store()



Warning

For the documentation skimmers: this is an explicit example on how not to use work functions. The correct implementation calls calculation functions to perform the computation

This time around the addition is correctly performed by a calculation function as it should, however, its result is multiplied by the work function itself and returned. Note that once again store had to be called explicitly on product to avoid the engine throwing a ValueError, which is only for the purpose of this example and should not be done in practice. The resulting provenance would look like the following:

Fig. 16 The provenance generated by the incorrect work function implementation that uses only a calculation function for the addition but performs the multiplication itself. The red cross is there to indicate that there is no actual connection between the intermediate sum D4 and the final result D5, even though the latter in reality derives from the former.

The generated provenance shows, that although the addition is explicitly represented because the work function called the calculation function, there is no connection between the sum and the final result. That is to say, there is no direct link between the sum D4 and the final result D5, as indicated by the red cross, even though we know that the final answer was based on the intermediate sum. This is a direct cause of the work function ‘creating’ new data and illustrates how, in doing so, the provenance of data creation is lost.

### Exit codes¶

To terminate the execution of a work function and mark it as failed, one simply has to return an exit code. The ExitCode class is constructed with an integer, to denote the desired exit status and an optional message When such as exit code is returned, the engine will mark the node of the work function as Finished and set the exit status and message to the value of the exit code. Consider the following example:

@workfunction
def exiting_workfunction():
from aiida.engine import ExitCode
return ExitCode(418, 'I am a teapot')


The execution of the work function will be immediately terminated as soon as the exit code is returned, and the exit status and message will be set to 418 and I am a teapot, respectively. Since no output nodes are returned, the WorkFunctionNode node will have no outputs and the value returned from the function call will be an empty dictionary.

## Work chains¶

The basic concept of the work chain has been explained elsewhere. This section will provide details on how a work chain can and should be implemented. A work chain is implemented by the WorkChain class. Since it is a sub class of the Process class, it shares all its properties. It will be very valuable to have read the section on working with generic processes before continuing, because all the concepts explained there will apply also to work chains.

Let’s continue with the example presented in the section on the concept of workchains, where we sum two integers and multiply the result with a third. We provided a very simple implementation in a code snippet, whose generated provenance graph, when executed, is shown in Fig. 13. For convenience we copy the snippet here once more:

# -*- coding: utf-8 -*-
from aiida.engine import WorkChain, calcfunction
from aiida.orm import Int

@calcfunction
return Int(x + y)

@calcfunction
def multiply(x, y):
return Int(x * y)

@classmethod
def define(cls, spec):
super().define(spec)
spec.input('x')
spec.input('y')
spec.input('z')
spec.outline(
cls.multiply,
cls.results,
)
spec.output('result')

def multiply(self):
self.ctx.product = multiply(self.ctx.sum, self.inputs.z)

def results(self):
self.out('result', self.ctx.product)


We will now got through the implementation step-by-step and go into more detail on the interface and best practices.

### Define¶

To implement a new work chain, simply create a new class that sub classes WorkChain. You can give the new class any valid python class name, but the convention is to have it end in WorkChain so that it is always immediately clear what it references. After having created a new work chain class, the first and most important method to implement is the define() method. This is a class method that allows the developer to define the characteristics of the work chain, such as what inputs it takes, what outputs it can generate, what potential exit codes it can return and the logical outline through which it will accomplish all this.

To implement the define method, you have to start with the following three lines:

@classmethod
def define(cls, spec):
super().define(spec)


where you replace AddAndMultiplyWorkChain with the actual name of your work chain. The @classmethod decorator indicates that this method is a class method 1 and not an instance method. The second line is the method signature and specified that it will receive the class itself cls and spec which will be an instance of the ProcessSpec. This is the object that we will use to define our inputs, outputs and other relevant properties of the work chain. The third and final line is extremely important, as it will call the define method of the parent class, in this case the WorkChain class.

Warning

If you forget to call super in the define method, your work chain will fail miserably!

### Inputs and outputs¶

With those formalities out of the way, you can start defining the interesting properties of the work chain through the spec. In the example you can see how the method input() is used to define multiple input ports, which document exactly which inputs the work chain expects. Similarly, output() is called to instruct that the work chain will produce an output with the label result. These two port creation methods support a lot more functionality, such as adding help string, validation and more, all of which is documented in detail in the section on ports and port namespace.

### Outline¶

The outline is what sets the work chain apart from other processes. It is a way of defining the higher-level logic that encodes the workflow that the work chain takes. The outline is defined in the define method through the outline(). It takes a sequence of instructions that the work chain will execute, each of which is implemented as a method of the work chain class. In the simple example above, the outline consists of three simple instructions: add, multiply, results. Since these are implemented as instance methods, they are prefixed with cls. to indicate that they are in fact methods of the work chain class. For that same reason, their implementation should take self as its one and only argument, as demonstrated in the example snippet.

The outline in this simple example is not particular interesting as it consists of three simple instructions that will be executed sequentially. However, the outline also supports various logical constructs, such as while-loops, conditionals and return statements. As usual, the best way to illustrate these constructs is by example. The currently available logical constructs for the work chain outline are:

• if, elif, else

• while

• return

To distinguish these constructs from the python builtins, they are suffixed with an underscore, like so while_. To use these in your work chain design, you will have to import them:

from aiida.engine import if_, while_, return_


The following example shows how to use these logical constructs to define the outline of a work chain:

spec.outline(
cls.intialize_to_zero,
while_(cls.n_is_less_than_hundred)(
if_(cls.n_is_multitple_of_three)(
cls.report_fizz,
).elif_(cls.n_is_multiple_of_five)(
cls.report_buzz,
).elif_(cls.n_is_multiple_of_three_and_five)(
cls.report_fizz_buzz,
).else_(
cls.report_n,
)
),
cls.increment_n_by_one,
)


This is an implementation (and an extremely contrived one at that) of the well known FizzBuzz 2 problem. The idea is that the program is supposed to print in sequence the numbers from zero to some limit, except when the number is a multiple of three Fizz is printed, for a multiple of five Buzz and when it is a multiple of both, the program should print FizzBuzz. Note how the syntax looks very much like that of normal python syntax. The methods that are used in the conditionals (between the parentheses of the while_ and if_ constructs) for example should return a boolean; True when the condition holds and False otherwise. The actual implementation of the outline steps themselves is now trivial:

def initialize_to_zero(self):
self.ctx.n = 0

def n_is_less_than_hundred(self):
return self.ctx.n < 100

def n_is_multiple_of_three(self):
return self.ctx.n % 3 == 0

def n_is_multiple_of_five(self):
return self.ctx.n % 5 == 0

def n_is_multiple_of_three_and_five(self):
return self.ctx.n % 3 == 0 and self.ctx.n % 5 == 0

def increment_n_by_one(self):
self.ctx.n += 1


The intention of this example is to show that with a well designed outline, a user only has to look at the outline to have a good idea what the work chain does and how it does it. One should not have to look at the implementation of the outline steps as all the important information is captured by the outline itself. Since the goal of a work chain should be to execute a very well defined task, it is the goal of the outline to capture the required logic to achieve that goal, in a clear and short yet not overly succint manner. The outline supports various logical flow constructs, such as conditionals and while loops, so where possible this logic should be expressed in the outline and not in the body of the outline functions. However, one can also go overboard and put too finely grained logical blocks into the outline, causing it to become bulky and difficult to understand.

A good rule of thumb in designing the outline is the following: before you start designing a work chain, define very clearly the task that it should carry out. Once the goal is clear, draw a schematic block diagram of the necessary steps and logical decisions that connect them, in order to accomplish that goal. Converting the resulting flow diagram in a one-to-one fashion into an outline, often results in very reasonable outline designs.

### Exit codes¶

There is one more property of a work chain that is specified through its process specification, in addition to its inputs, outputs and outline. Any work chain may have one to multiple failure modes, which are modeled by exit codes. A work chain can be stopped at any time, simply by returning an exit code from an outline method. To retrieve an exit code that is defined on the spec, one can use the exit_codes() property. This returns an attribute dictionary where the exit code labels map to their corresponding exit code. For example, with the following process spec:

spec = ProcessSpec()
spec.exit_code(418, 'ERROR_I_AM_A_TEAPOT', 'the process had an identity crisis')


To see how exit codes can be used to terminate the execution of work chains gracefully, refer to the section Aborting and exit codes.

### Launching work chains¶

The rules for launching work chains are the same as those for any other process, which are detailed in this section. On top of those basic rules, there is one peculiarity in the case of work chains when submitting to the daemon. When you submit a WorkChain over the daemon, or any other process for that matter, you need to make sure that the daemon can find the class when it needs to load it. Registering your class through the plugin system with a designated entry point is one way to make sure that the daemon will be able to find it. If, however, you simply have a test class and do not want to go through the effort of creating an entry point for it, you should make sure that the module where you define the class is in the python path. Additionally, make sure that the definition of the work chain is not in the same file from which you submit it, or the engine won’t be able to load it.

### Context¶

In the simplest work chain example presented in the introductory section, we already saw how the context can be used to persist information during the execution of a work chain and pass it between outline steps. The context is essentially a data container, very similar to a dictionary that can hold all sorts of data. The engine will ensure that its contents are saved and persisted in between steps and when the daemon shuts down or restarts. A trivial example of this would be the following:

def step_one(self):
self.ctx.some_variable = 'store me in the context'

def step_two(self):
assert self.ctx.some_variable == 'store me in the context'


In the step_one outline step we store the string 'store me in the context' in the context, which can be addressed as self.ctx, under the key some_variable. Note that for the key you can use anything that would be a valid key for a normal python dictionary. In the second outline step step_two, we can verify that the string was successfully persisted, by checking the value stored in the context self.ctx.some_variable.

Warning

Any data that is stored in the context has to be serializable.

This was just a simple example to introduce the concept of the context, however, it really is one of the more important parts of the work chain. The context really becomes crucial when you want to submit a calculation or another work chain from within the work chain. How this is accomplished, we will show in the next section.

### Submitting sub processes¶

One of the main tasks of a WorkChain will be to launch other processes, such as a CalcJob or another WorkChain. How to submit processes was explained in another section and is accomplished by using the submit() launch function. However, when submitting a sub process from within a work chain, this should not be used. Instead, the Process class provides its own submit() method. If you do, you will be greeted with the exception:

InvalidOperation: 'Cannot use top-level submit from within another process, use self.submit instead'


The only change you have to make is to replace the top-level submit method with the built-in method of the process class:

def submit_sub_process(self)
node = self.submit(SomeProcess, **inputs)  # Here we use self.submit and not submit from aiida.engine


The self.submit method has the exact same interface as the global aiida.engine.launch.submit launcher. When the submit method is called, the process is created and submitted to the daemon, but at that point it is not yet done. So the value that is returned by the submit call is not the result of the submitted process, but rather it is the process node that represents the execution of the process in the provenance graph and acts as a future. We somehow need to tell the work chain that it should wait for the sub process to be finished, and the future to resolve, before it continues. To do so, however, control has to be returned to the engine, which can then, when the process is completed, call the next step in the outline, where we can analyse the results. The snippet above already revealed that this is accomplished by returning an instance of the ToContext class.

#### To context¶

In order to store the future of the submitted process, we can store it in the context with a special construct that will tell the engine that it should wait for that process to finish before continuing the work chain. To illustrate how this works, consider the following minimal example:

# -*- coding: utf-8 -*-
from aiida.engine import WorkChain, ToContext

class SomeWorkChain(WorkChain):

@classmethod
def define(cls, spec):
super().define(spec)
spec.outline(
cls.submit_workchain,
cls.inspect_workchain,
)

def submit_workchain(self):
future = self.submit(SomeWorkChain)

def inspect_workchain(self):
assert self.ctx.workchain.is_finished_ok


As explained in the previous section, calling self.submit for a given process that you want to submit, will return a future. To add this future to the context, we can not access the context directly as explained in the context section, but rather we need to use the class ToContext. This class has to be imported from the aiida.engine module. To add the future to the context, simply construct an instance of ToContext, passing the future as a keyword argument, and returning it from the outline step. The keyword used, workchain in this example, will be the key used under which to store the node in the context once its execution has terminated. Returning an instance of ToContext signals to the engine that it has to wait for the futures contained within it to finish execution, store their nodes in the context under the specified keys and then continue to the next step in the outline. In this example, that is the inspect_workchain method. At this point we are sure that the process, a work chain in this case, has terminated its execution, although not necessarily successful, and we can continue the logic of the work chain.

Warning

Using the ToContext construct alone is not enough to tell the engine that it should wait for the sub process to finish. There needs to be at least another step in the outline to follow the step that added the awaitables. If there is no more step to follow, according to the outline, the engine interprets this as the work chain being done and so it will not wait for the sub process to finish. Think about it like this: if there is not even a single step to follow, there is also nothing the work chain could do with the results of the sub process, so there is no point in waiting.

Sometimes one wants to launch not just one, but multiple processes at the same time that can run in parallel. With the mechanism described above, this will not be possible since after submitting a single process and returning the ToContext instance, the work chain has to wait for the process to be finished before it can continue. To solve this problem, there is another way to add futures to the context:

# -*- coding: utf-8 -*-
from aiida.engine import WorkChain

class SomeWorkChain(WorkChain):

@classmethod
def define(cls, spec):
super().define(spec)
spec.outline(
cls.submit_workchains,
cls.inspect_workchains,
)

def submit_workchains(self):
for i in range(3):
future = self.submit(SomeWorkChain)
key = 'workchain_{}'.format(i)
self.to_context(**{key: future})

def inspect_workchains(self):
for i in range(3):
key = 'workchain_{}'.format(i)
assert self.ctx[key].is_finished_ok


Here we submit three work chains in a for loop in a single outline step, but instead of returning an instance of ToContext, we call the to_context() method. This method has exactly the same syntax as the ToContext class, except it is not necessary to return its value, so we can call it multiple times in one outline step. Under the hood the functionality is also the same as the ToContext class. At the end of the submit_workchains outline step, the engine will find the futures that were added by calling to_context and will wait for all of them to be finished. The good thing here is that these three sub work chains can be run in parallel and once all of them are done, the parent work chain will go to the next step, which is inspect_workchains. There we can find the nodes of the work chains in the context under the key that was used as the keyword argument in the to_context call in the previous step.

Since we do not want the subsequent calls of to_context to override the previous future, we had to create unique keys to store them under. In this example, we chose to use the index of the for-loop. The name carries no meaning and is just required to guarantee unique key names. This pattern will occur often where you will want to launch multiple work chains or calculations in parallel and will have to come up with unique names. In essence, however, you are really just creating a list and it would be better to be able to create a list in the context and simply append the future to that list as you submit them. How this can be achieved is explained in the next section.

#### Appending¶

When you want to add a future of a submitted sub process to the context, but append it to a list rather than assign it to a key, you can use the append_() function. Consider the example from the previous section, but now we will use the append_ function instead:

# -*- coding: utf-8 -*-
from aiida.engine import WorkChain, append_

class SomeWorkChain(WorkChain):

@classmethod
def define(cls, spec):
super().define(spec)
spec.outline(
cls.submit_workchains,
cls.inspect_workchains,
)

def submit_workchains(self):
for i in range(3):
future = self.submit(SomeWorkChain)
self.to_context(workchains=append_(future))

def inspect_workchains(self):
for workchain in self.ctx.workchains:
assert workchain.is_finished_ok


Notice that in the submit_workchains step we no longer have to generate a unique key based on the index but we simply wrap the future in the append_ function and assign it to the generic key workchains. The engine will see the append_ function and instead of assigning the node corresponding to the future to the key workchains, it will append it to the list stored under that key. If the list did not yet exist, it will automatically be created. The self.ctx.workchains now contains a list with the nodes of the completed work chains, with the same order as they had been inserted, and so in the inspect_workchains step we can simply iterate over it to access all of them. Note that the use of append_ is not just limited to the to_context method. You can also use it in exactly the same way with ToContext to append a process to a list in the context in multiple outline steps.

### Reporting¶

During the execution of a WorkChain, we may want to keep the user abreast of its progress and what is happening. For this purpose, the WorkChain implements the report() method, which functions as a logger of sorts. It takes a single argument, a string, that is the message that needs to be reported:

def submit_calculation(self):
self.report('here we will submit a calculation')


This will send that message to the internal logger of python, which will cause it to be picked up by the default AiiDA logger, but it will also trigger the database log handler, which will store the message in the database and link it to the node of the work chain. This allows the verdi process report command to retrieve all those messages that were fired using the report method for a specific process. Note that the report method, in addition to the pk of the work chain, will also automatically record the name of the work chain and the name of the outline step in which the report message was fired. This information will show up in the output of verdi process report, so you never have to explicitly reference the work chain name, outline step name or date and time in the message itself.

It is important to note that the report system is a form of logging and as such has been designed to be read by humans only. That is to say, the report system is not designed to pass information programmatically by parsing the log messages.

### Aborting and exit codes¶

At the end of every outline step, the return value will be inspected by the engine. If a non-zero integer value is detected, the engine will interpret this as an exit code and will stop the execution of the work chain, while setting its process state to Finished. In addition, the integer return value will be set as the exit_status of the work chain, which combined with the Finished process state will denote that the worchain is considered to be Failed, as explained in the section on the process state. This is useful because it allows a workflow designer to easily exit from a work chain and use the return value to communicate programmatically the reason for the work chain stopping.

We assume that you have read the section on how to define exit code through the process specification of the work chain. Consider the following example work chain that defines such an exit code:

spec.exit_code(400, 'ERROR_CALCULATION_FAILED', 'the child calculation did not finish successfully')


Now imagine that in the outline, we launch a calculation and in the next step check whether it finished successfully. In the event that the calculation did not finish successfully, the following snippet shows how you can retrieve the corresponding exit code and abort the WorkChain by returning it:

def submit_calculation(self):
inputs = {'code': code}
future = self.submit(SomeCalcJob, **inputs)

def inspect_calculation(self):
if not self.ctx.calculation.is_finished_ok:
self.report('the calculation did not finish successfully, there is nothing we can do')
return self.exit_codes.ERROR_CALCULATION_FAILED

self.report('the calculation finished successfully')


In the inspect_calculation outline, we retrieve the calculation that was submitted and added to the context in the previous step and check if it finished successfully through the property is_finished_ok. If this returns False, in this example we simply fire a report message and return the exit code corresponding to the label ERROR_CALCULATION_FAILED. Note that the specific exit code can be retrieved through the WorkChain property exit_codes. This will return a collection of exit codes that have been defined for that WorkChain and any specific exit code can then be retrieved by accessing it as an attribute. Returning this exit code, which will be an instance of the ExitCode class, will cause the work chain to be aborted and the exit_status and exit_message to be set on the node, which were defined in the spec.

Note

The notation self.exit_codes.ERROR_CALCULATION_FAILED is just syntactic sugar to retrieve the ExitCode instance that was defined in the spec with that error label. Constructing your own ExitCode directly and returning that from the outline step will have exactly the same effect in terms of aborting the work chain execution and setting the exit status and message. However, it is strongly advised to define the exit code through the spec and retrieve it through the self.exit_codes collection, as that makes it easily retrievable through the spec by the caller of the work chain.

The message attribute of an ExitCode can also be a string that contains placeholders. This is useful when the exit code’s message is generic enough to a host of situations, but one would just like to parameterize the exit message. To concretize the template message of an exit code, simply call the format() method and pass the parameters as keyword arguments:

.. code:: python


exit_code_template = ExitCode(450, ‘the parameter {parameter} is invalid.’) exit_code_concrete = exit_code_template.format(parameter=’some_specific_key’)

This concept can also be applied within the scope of a process. In the process spec, we can declare a generic exit code whose exact message should depend on one or multiple parameters:

.. code:: python


spec.exit_code(450, ‘ERROR_INVALID_PARAMETER, ‘the parameter {parameter} is invalid.’)

Through the self.exit_codes collection of a WorkChain, this generic can be easily customized as follows:

def inspect_calculation(self):
return self.exit_codes.ERROR_INVALID_PARAMETER.format(parameter='some_specific_key')


This is no different than the example before, because self.exit_codes.ERROR_INVALID_PARAMETER simply returns an instance of ExitCode, which we then call format on with the substitution parameters.

In conclusion, the best part about using exit codes to abort a work chain’s execution, is that the exit status can now be used programmatically, by for example a parent work chain. Imagine that a parent work chain submitted this work chain. After it has terminated its execution, the parent work chain will want to know what happened to the child work chain. As already noted in the report section, the report messages of the work chain should not be used. The exit status, however, is a perfect way. The parent work chain can easily request the exit status of the child work chain through the exit_status property, and based on its value determine how to proceed.

### Modular workflow design¶

When creating complex workflows, it is a good idea to split them up into smaller, modular parts. At the lowest level, each workflow should perform exactly one task. These workflows can then be wrapped together by a “parent” workflow to create a larger logical unit.

In order to make this approach manageable, it needs to be as simple as possible to glue together multiple workflows in a larger parent workflow. One of the tools that AiiDA provides to simplify this is the ability to expose the ports of another work chain.

#### Exposing inputs and outputs¶

Consider the following example work chain, which simply takes a few inputs and returns them again as outputs:

# -*- coding: utf-8 -*-
from aiida.orm import Bool, Float, Int
from aiida.engine import WorkChain

class ChildWorkChain(WorkChain):

@classmethod
def define(cls, spec):
super().define(spec)
spec.input('a', valid_type=Int)
spec.input('b', valid_type=Float)
spec.input('c', valid_type=Bool)
spec.outline(cls.do_run)
spec.output('d', valid_type=Int)
spec.output('e', valid_type=Float)
spec.output('f', valid_type=Bool)

def do_run(self):
self.out('d', self.inputs.a)
self.out('e', self.inputs.b)
self.out('f', self.inputs.c)


As a first example, we will implement a thin wrapper workflow, which simply forwards its inputs to ChildWorkChain, and forwards the outputs of the child to its outputs:

# -*- coding: utf-8 -*-
from aiida.engine import ToContext, WorkChain, run
from child import ChildWorkChain

class SimpleParentWorkChain(WorkChain):

@classmethod
def define(cls, spec):
super().define(spec)
spec.expose_inputs(ChildWorkChain)
spec.expose_outputs(ChildWorkChain)
spec.outline(cls.run_child, cls.finalize)

def run_child(self):
child = self.submit(ChildWorkChain, **self.exposed_inputs(ChildWorkChain))

def finalize(self):
self.out_many(
self.exposed_outputs(self.ctx.child, ChildWorkChain)
)


In the define method of this simple parent work chain, we use the expose_inputs() and expose_outputs(). This creates the corresponding input and output ports in the parent work chain. Additionally, AiiDA remembers which inputs and outputs were exposed from that particular work chain class. This is used when calling the child in the run_child method. The exposed_inputs() method returns a dictionary of inputs that the parent received which were exposed from the child, and so it can be used to pass these on to the child. Finally, in the finalize method, we use exposed_outputs() to retrieve the outputs of the child which were exposed to the parent. Using out_many(), these outputs are added to the outputs of the parent work chain. This work chain can now be run in exactly the same way as the child itself:

#!/usr/bin/env runaiida
# -*- coding: utf-8 -*-

from aiida.orm import Bool, Float, Int
from aiida.engine import run
from simple_parent import SimpleParentWorkChain

if __name__ == '__main__':
result = run(SimpleParentWorkChain, a=Int(1), b=Float(1.2), c=Bool(True))
print(result)
# {'e': 1.2, 'd': 1, 'f': True}


Next, we will see how a more complex parent work chain can be created by using the additional features of the expose functionality. The following work chain launches two children. These children share the input a, but have different b and c. The output e will be taken only from the first child, whereas d and f are taken from both children. In order to avoid name conflicts, we need to create a namespace for each of the two children, where the inputs and outputs which are not shared are stored. Our goal is that the workflow can be called as follows:

#!/usr/bin/env runaiida
# -*- coding: utf-8 -*-

from aiida.orm import Bool, Float, Int
from aiida.engine import run
from complex_parent import ComplexParentWorkChain

if __name__ == '__main__':
result = run(
ComplexParentWorkChain,
a=Int(1),
child_1=dict(b=Float(1.2), c=Bool(True)),
child_2=dict(b=Float(2.3), c=Bool(False))
)
print(result)
# {
#     'e': 1.2,
#     'child_1.d': 1, 'child_1.f': True,
#     'child_2.d': 1, 'child_2.f': False
# }


This is achieved by the following workflow. In the next section, we will explain each of the steps.

# -*- coding: utf-8 -*-
from aiida.engine import ToContext, WorkChain, run

from child import ChildWorkChain

class ComplexParentWorkChain(WorkChain):
@classmethod
def define(cls, spec):
super().define(spec)
spec.expose_inputs(ChildWorkChain, include=['a'])
spec.expose_inputs(ChildWorkChain, namespace='child_1', exclude=['a'])
spec.expose_inputs(ChildWorkChain, namespace='child_2', exclude=['a'])
spec.outline(cls.run_children, cls.finalize)
spec.expose_outputs(ChildWorkChain, include=['e'])
spec.expose_outputs(ChildWorkChain, namespace='child_1', exclude=['e'])
spec.expose_outputs(ChildWorkChain, namespace='child_2', exclude=['e'])

def run_children(self):
child_1_inputs = self.exposed_inputs(ChildWorkChain, namespace='child_1')
child_2_inputs = self.exposed_inputs(ChildWorkChain, namespace='child_2', agglomerate=False)
child_1 = self.submit(ChildWorkChain, **child_1_inputs)
child_2 = self.submit(ChildWorkChain, a=self.inputs.a, **child_2_inputs)

def finalize(self):
self.out_many(
self.exposed_outputs(
self.ctx.child_1,
ChildWorkChain,
namespace='child_1'
)
)
self.out_many(
self.exposed_outputs(
self.ctx.child_2,
ChildWorkChain,
namespace='child_2',
agglomerate=False
)
)


First of all, we want to expose the a input and the e output at the top-level. For this, we again use expose_inputs() and expose_outputs(), but with the optional keyword include. This specifies a list of keys, and only inputs or outputs which are in that list will be exposed. So by passing include=['a'] to expose_inputs(), only the input a is exposed.

Additionally, we want to expose the inputs b and c (outputs d and f), but in a namespace specific for each of the two children. For this purpose, we pass the namespace parameter to the expose functions. However, since we now shouldn’t expose a (e) again, we use the exclude keyword, which specifies a list of keys that will not be exposed.

When calling the children, we again use the exposed_inputs() method to forward the exposed inputs. Since the inputs b and c are now in a specific namespace, we need to pass this namespace as an additional parameter. By default, exposed_inputs() will search through all the parent namespaces of the given namespace to search for input, as shown in the call for child_1. If the same input key exists in multiple namespaces, the input in the lowest namespace takes precedence. It’s also possible to disable this behavior, and instead search only in the explicit namespace that was passed. This is done by setting agglomerate=False, as shown in the call to child_2. Of course, we then need to explicitly pass the input a.

Finally, we use exposed_outputs() and out_many() to forward the outputs of the children to the outputs of the parent. Again, the namespace and agglomerate options can be used to select which outputs are returned by the exposed_outputs() method.

Footnotes

1

https://docs.python.org/3.5/library/functions.html#classmethod

2

https://en.wikipedia.org/wiki/Fizz_buzz