Looping Sections

What are loops? Refer to the end of this section.

As an integral extension of Test Parameters data-driven testing concept, aetest also supports section looping: reusing section code body by providing it with different parameters during each loop iteration.

The following describes each section and their loop capability and behaviors:


Common setup and cleanup sections are unique within each testscript. They are run only once per testscript execution, and are not loopable.


Subsections within CommonSetup and CommonCleanup are loopable. When a subsection is marked for looping, each of its iterations is reported as a new subsection.


Testcases are loopable. Each iteration of a looping Testcase is reported individually as new testcase instances with different uid. When a Testcase is looped, all of its contents (setup, tests and cleanup) are run fully per each iteration.


Setup and cleanup sections within each testcase is unique, and are run only once per Testcase. They cannot be looped individually, but if their parent Testcase is looped, then they are run once per Testcase iteration.


Test sections within Testcase are loopable individually. Each iteration has its own unique id, and is reported as a new test section. When a looping test section’s parent Testcase is also looped, the resulting loops are multiplicative. Eg: if a testcase is looped 2x, and contains a test that is also looped 2x, that test would loop 2x per testcase loop iteration.


in other words, subsection, Testcase and test sections are the only loopable sections.

Defining Loops

Sections are marked for looping when they are decorated with @loop, and its looping parameters provided as decorator arguments. During runtime, when aetest infrastructure detects looped section code, their corresponding section object is then instantiated once for each of its iterations.

# Example
# -------
#   defining loops on sections

from pyats import aetest

# defining a common setup section
# contains a subsection that is looped twice.
class CommonSetup(aetest.CommonSetup):

    # defining a subsection
    # this subsection is marked to be looped twice
    # the first time having a uid of "subsection_one", and
    # the second time having a uid of "subsection_two"
    @aetest.loop(uids=['subsection_one', 'subsection_two'])
    def looped_subsection(self):

# defining a testcase that loops
# this testcase also contains a test section that is looped twice
@aetest.loop(uids=['testcase_one', 'testcase_two'])
class Testcase(aetest.Testcase):

    # setup section of this testcase is run once
    # every time the testcase is looped.
    def setup(self):

    # looped test section
    # both iterations are run per testcase iteration
    @aetest.loop(uids=['test_one', 'test_two'])
    def test(self):

    # cleanup section of this testcase is run once
    # every time the testcase is looped.
    def cleanup(self):

# this testscript's resulting sections would look like this below
# SECTIONS/TESTCASES                                             RESULT
# ----------------------------------------------------------------------
#  .
#  |-- CommonSetup                                               PASSED
#  |   |-- subsection_one                                        PASSED
#  |   `-- subsection_two                                        PASSED
#  |-- testcase_one                                              PASSED
#  |   |-- setup                                                 PASSED
#  |   |-- test_one                                              PASSED
#  |   |-- test_two                                              PASSED
#  |   `-- cleanup                                               PASSED
#  `-- testcase_two                                              PASSED
#      |-- setup                                                 PASSED
#      |-- test_one                                              PASSED
#      |-- test_two                                              PASSED
#      `-- cleanup                                               PASSED

As shown above, the minimum requirement to loop a section (eg, to run its code 1+ times) is to decorate the section with @loop, and provide a list of loop iteration uids using uids argument. This controls the number of iterations this section is looped: each unique item in the uids list generates to a new section with that uid.

When @loop is used on a @subsection or @test, the section method is effectively decorated twice, and even though the order does not matter, it make more sense to use @loop as the outermost decorator, signifying that this method is first marked as a section, then this section is looped.


decorators are executed in the order of “innermost” to “outermost”.

In addition, in an effort to make the script more aesthetically pleasing, aetest also features a shortcut to avoid the double decorators: @subsection.loop and @test.loop.

# Example
# -------
#   demonstration the double decorator shortcut for test and subsections

from pyats import aetest

class CommonSetup(aetest.CommonSetup):

    # marking this as both a subsection, and being looped
    @aetest.subsection.loop(uids=['subsection_one', 'subsection_two'])
    def looped_subsection(self):

class Testcase(aetest.Testcase):

    # marking this as both a test section and being looped
    @aetest.test.loop(uids =['test_one', 'test_two'])
    def test(self):


@subsection.loop and @test.loop are convenience features. They are not a python decorator compression technique. .loop is an attribute to subsection/test that are implemented in aetest. It is effectively a new decorator that shoots two birds with one stone.


python @decorators are evaluated at import time. Thus, decorator arguments may only be static. If you need to reference runtime and/or dynamic information information as part of your loop declaration, eg, accessing parameters & etc, refer to Dynamic Loop Marking.

Loop Parameters

Looping the same section again and again is not very useful. Even if each section has a unique uid as demonstrated above, the usefulness of a test that repeatedly perform the same actions is questionable. This is where loop parameters comes in.

Loop parameters feature allows each loop iteration to receive new, distinct Test Parameters. These parameters are specified as part of the @loop decorator, processed and propagated to each section instance as their local parameters. Combined with Parameters as Function Arguments feature, each looped section is then driven to potentially do something different.

# Example
# -------
#   loop parameters demonstration

from pyats import aetest

# loop this testcast with a loop parameter named "a"
# and set it to value 2 for the first iteration,
# and 3 for the second iteration
@aetest.loop(a=[2, 3])
class Testcase(aetest.Testcase):

    # loop this test with loop parameter named "b"
    # and set it to 8 for the first iteration, 9 for the second.
    @aetest.test.loop(b=[8, 9])
    def test(self, a, b):
        # this test prints the exponential of two inputs, a and b
        print("%s ^ %s = %s" % (a, b, a**b))

# the output of the testcase would look like this:
#   2 ^ 8 = 256
#   2 ^ 9 = 512
#   3 ^ 8 = 6561
#   3 ^ 9 = 19683
# and since no uids were provided as part of the loop decorator, new uids
# are generated based on the original section name and the input parameters
#  SECTIONS/TESTCASES                                               RESULT
# --------------------------------------------------------------------------
#  .
#  |-- Testcase[a=2]                                                PASSED
#  |   |-- test[b=8]                                                PASSED
#  |   `-- test[b=9]                                                PASSED
#  `-- Testcase[a=3]                                                PASSED
#      |-- test[b=8]                                                PASSED
#      `-- test[b=9]                                                PASSED

In effect, loop parameters allows users to create and/or modify the looped section’s local parameters on the fly, per iteration. It is an extension of the dynamic parameter concept, where section parameters are being generated and fed to each section during runtime.

The use of loop parameters also makes uids argument optional: if uids are not provided, the infrastructure generates unique section uids by combining the original section name with each of its current loop parameters as postfix in square backets. Otherwise, the provided uids are used as section uids.

There are two methods of providing loop parameters to the @loop decorator:

  • by providing a list of parameters, and a list of parameter values for each iteration (eg, using args and argvs)

  • by providing each parameter as a keyword argument, and a list of its corresponding argument values. (eg, a=[1, 2, 3], b=[4, 5, 6])

# Example
# -------
#   providing loop parameters

from pyats import aetest

class Testcase(aetest.Testcase):

    # loop this test with arguments "a", "b", and "c"
    # provide all of its iteration arguments together using method one
    # the positions of each value in argvs corresponds to its args name
    @aetest.test.loop(args=('a', 'b', 'c'),
                      argvs=((1, 2, 3),
                             (4, 5, 6)))
    def test_one(self, a, b, c):
        print("a=%s, b=%s, c=%s" % (a, b, c))

    # loop this test with the same arguments as above, but
    # provide each of its iteration arguments independently using method two
    def test_two(self, a, b, c):
        print("a=%s, b=%s, c=%s" % (a, b, c))

# testcase output:
#   a=1, b=2, c=3
#   a=4, b=5, c=6
#   a=1, b=2, c=3
#   a=4, b=5, c=6
#  SECTIONS/TESTCASES                                               RESULT
# --------------------------------------------------------------------------
#  .
#  `-- Testcase                                                     PASSED
#      |-- test_one[a=1,b=2,c=3]                                    PASSED
#      |-- test_one[a=4,b=5,c=6]                                    PASSED
#      |-- test_two[a=1,b=2,c=3]                                    PASSED
#      `-- test_two[a=4,b=5,c=6]                                    PASSED

As shown above, there were no difference in the outcome of the results. The only difference was how the loop parameters were provided. One method may be superior to the other depending on the situation, the number of arguments, etc.

When using loop parameters, the following rules determines the actual number of iterations:

  • if uids were provided, the number of iterations is equal to the number of uids provided.

    • if the number of parameter values exceeds the number of uids, all extra values are discarded.

  • if uids are not provided, the number of iterations is equal to the number of loop parameter values. Eg, if @loop(a=[1,2,3]), then there would be 3 loop instances, each taking on one distinct value: a=1, a=2, a=3.

    • if there are multiple parameters and the number of their values do not agree, or if the number of parameter values is less than the number of provided uids, a filler is used to fill empty spots. filler defaults to None, and only 1 filler can be provided.

# Example
# -------
#   loop parameter combinations
#   (pseudo code for demonstration only)

from pyats.aetest import loop

# loop with 2 iterations using uids argument
# ------------------------------------------
#   iteration 1: uid='id_one'
#   iteration 2: uid='id_two'
@loop(uids=['id_one', 'id_two'])

# loop with 2 iterations using parameters argument
# ------------------------------------------------
#   iteration 1: a=1, b=4
#   iteration 2: a=2, b=5
@loop(a = [1, 2], b = [4, 5])
# same as above, using args and argvs
@loop(args=['a', 'b'], argvs=[(1, 4), (2, 5)])

# loop with 2 iterations, and extra arguments are discarded due to uids
# ---------------------------------------------------------------------
#   iteration 1: uid='id_one', a=1, b=2
#   iteration 2: uid='id_two', a=3, b=4
# extra argument values 5/6 are discarded because there are no matching uids
@loop(uids=['id_one', 'id_two'],
      args=['a', 'b'],
      argvs=[(1, 2),
             (3, 4),
             (5, 6)])
# same example as above but using per-parameter values
@loop(uids=['id_one', 'id_two'],
      a=[1, 3, 5], b=[2, 4, 6])

# loop with 3 iterations, and their number of parameters values do not agree
# --------------------------------------------------------------------------
#   iteration 1: a=1, b=4
#   iteration 2: a=2, b=5
#   iteration 3: a=3, b=None ---> default filler comes in to fill the blanks
@loop(a=[1, 2, 3], b=[4, 5])
# same as above, using args and argvs
@loop(args=['a', 'b'],
      argvs=[(1, 4),
             (2, 5),
             (3, )])

# loop with more uids than parameters, and custom filler
# ------------------------------------------------------
#   iteration 1: uid='id_one', a=1, b=3
#   iteration 2: uid='id_two', a=2, b=4
#   iteration 1: uid='id_three', a=999, b=999  ---> custom filler
@loop(uids = ['id_one', 'id_two', 'id_three'],
      a = [1, 2], b = [3, 4], filler = 999)
# same as above, using args and argvs
@loop(uids=['id_one', 'id_two', 'id_three'],
      args=['a', 'b'], argvs=[(1, 3), (2, 3)], filler=999)

Advanced Loop Usages

Arguments to the @loop decorator may also be callable, iterable, or a generator. The infrastructure is able to distinguish and treat each as you would normally expect it to:

  • if an argument value is a callable, it is called, and its returns are then used as the actual loop argument value.

  • if an argument value is an iterable or a generator, the loop engine picks only one element from it at a time to build the next iteration, until it is exhausted.

# Example
# -------
#   demonstrating advanced loop parameter behaviors

from pyats import aetest

# defining a function
# functions are callable
def my_function():
    value = [1, 2, 3]
    print("returning %s" % value)
    return value

# defining a generator
def my_generator():
    for i in [4, 5, 6]:
        print('generating %s' % i)
        yield i

class Testcase(aetest.Testcase):

    # creating test section with parameter "a" as a function
    # note that the function object is passed, not its values
    def test_one(self, a):
        print("a = %s" % a)

    # creating a test section with parameter "b" as a generator
    # note that the generator is a result of calling my_generator(), not
    # the function itself.
    def test_two(self, b):
        print('b = %s' % b)

# the output of the testcase would be:
#   returning [1, 2, 3]
#   a = 1
#   a = 2
#   a = 3
#   generating 4
#   b = 4
#   generating 5
#   b = 5
#   generating 6
#   b = 6

In the above example, pay close attention to the output lines:

  • Callable arguments are called and converted into their return values before their looped sections are created and run.

  • Iterators and generators are only queried before the next section needs to be created.

This behavior enables the use of custom generator as input values to your loop parameters. For example, a generator state machine that queries the current testbed device status and creates iterations based on that information. Since the generator is not polled until right before the next iteration, your custom function is only run in-between test sections, thus dynamically generating the loop iterations based current test environments.

Dynamic Loop Marking

So far, all loop examples focused on defining @loop directly within the testscripts. Eg, the @loop decorators are coded as part of the testscript. In addition, it is also possible to dynamically mark sections for looping during runtime, eg, creating loops based on information that is only available during a script run. To do this, use the loop.mark() function.

# Example
# -------
#   dynamically marking sections for looping

from pyats import aetest

class Testcase(aetest.Testcase):

    def setup(self):
        # mark the next test for looping
        # provide it with two unique test uids.
        # (self.simple_test is the next test method)
        aetest.loop.mark(self.simple_test, uids=['test_one', 'test_two'])

    # note: the simple_test section is not directly marked for looping
    # instead, during runtime, its testcase's setup section marks it for
    # looping dynamically.

    def simple_test(self, section):
        # print the current section uid
        # by using the internal parameter "section"
        print("current section: %s" % section.uid)

# output of this testcase
#   current section: test_one
#   current section: test_two
#  SECTIONS/TESTCASES                                                RESULT
# --------------------------------------------------------------------------
#  .
#  `-- Testcase                                                      PASSED
#      |-- setup                                                     PASSED
#      |-- test_one                                                  PASSED
#      `-- test_two                                                  PASSED

loop.mark() arguments & behaviors (including loop parameters & etc) are exactly identical to its sibling @loop decorator, with the only exception that its first input argument must be the target section method/class. Eg: loop.mark(Testcase_Two, a=[1,2,3]).

The benefit of this approach is simple: dynamic information, parameters and variables such as Script Arguments, parent etc, are only available during runtime. This information and its corresponding variables are not available when the script is written, and delaying variable references (while using @loop decorator) in Python is very difficult, if not impossible.

Loop Internals

The previous sections focused on the “how to use” aspect of aetest looping functionality. From here onwards, we’ll dig deeper into loop internals, look at how it functions, and how to deviate from its default behaviors.

The aetest looping behavior & how its arguments are processed is actually highly customizable. This was not highlighted in previous sections for the sake of serializing the training & simplifying the learning curve.

In reality, consider @loop decorator and loop.mark() function as only markers: they only mark the given section for looping. The details (parameters) of each iterations is actually generated from loop generators, where all arguments to @loop and loop.mark() propagates to. Eg:

# Example
# -------
#   pseudo code demonstrating @loop decorator functionality

# what the loop decorator definition sort-of looks like
# note where the generator defaults to "DefaultLooper"
def loop(generator=DefaultLooper, *args, **kwargs):

    # the actual loop generator gets called with all of the arguments
    # to loop decorator, and generates each section iteration
    return generator(*args, **kwargs)

# pseudo code here onwards, demonstrating internals
# -------------------------------------------------
# during runtime, the looped is expanded to create each iteration

for iteration in loop(*args, **kwargs):
    # create a section from iteration information and run it
    # ...

    # eg, instantiate Subsection
    subsection = Subsection(uid=iteration.uid,

    # and add to common setup's subsections list

    # etc ...

Behind the scenes, loop generators are the actual classes that does the heavy lifting: creating each iteration based on @loop and loop.mark() decorator arguments. Loop generators are iterable. Each of its returned member is an instance of Iteration class, containing the uid & parameters information unique to this loop, and used by the infrastructure to create the next section instance.

Iteration class (collections.namedtuple)




iteration uid


a dictionary of Test Parameters to be applied to the next looped section

In other words, loop generator is the object that ultimately controls how loops are generated, and what parameters each iteration is associated with. The looping behavior and arguments described in topics above are actually that of DefaultLooper, the default loop generator provided by aetest loop infrastructure. Its features are sufficient for most use cases. However, if you wish to customize loop behavior, it is possible to extend and/or override it.

# Example
# -------
#   demonstrating how to write and pass your own loop generator

# loop generators must return Iterations
from pyats import aetest
from pyats.aetest.loop import Iteration

# let's write a custom loop generator
# it generates integers between a and b as loop iterations
# and pass the integer as "number" parameter of the executed section.
# each iteration uid is named "iteration_uid" + number
class DemoGenerator(object):

    # at minimum, the loop generator needs to accept an argument called
    # "loopee", which is the actual object being looped. This allows the
    # loop generator to know what it is looping on, and build information
    # based on it.
    # in this example, we're ignoring that argument, as our loop genrator
    # is simple and straightforward.
    def __init__(self, loopee, a, b):
        self.numbers = list(range(a, b))

    def __iter__(self):
        for i in self.numbers:
            # each generated member is an instance of Iteration
            # each Iteration must have a unique id
            # and all of its parameters stored in a dictionary
            yield Iteration(uid='iteration_uid_%s' % i,
                            parameters={'number': i})

# this loop generator can be used as @loop and loop.mark() argument.
# let's define a looped testcase with it.

# looping this testcase with custom generator, and a=1, b=5
@aetest.loop(generator=DemoGenerator, a=1, b=5)
class Testcase(aetest.Testcase):

    # since our generator generates a parameter named "number"
    # let's print it in this simple test.
    def test(self, number):
        print('current number: %s' % number)

# output of this testcase
#   current number: 1
#   current number: 2
#   current number: 3
#   current number: 4
#  SECTIONS/TESTCASES                                                RESULT
# --------------------------------------------------------------------------
#  .
#  |-- iteration_uid_1                                               PASSED
#  |   `-- test                                                      PASSED
#  |-- iteration_uid_2                                               PASSED
#  |   `-- test                                                      PASSED
#  |-- iteration_uid_3                                               PASSED
#  |   `-- test                                                      PASSED
#  `-- iteration_uid_4                                               PASSED
#      `-- test                                                      PASSED


the above examples may be simple, the demonstrated underlying principles are not.

“Do not try and bend the spoon. That’s impossible. Instead… only try to realize the truth…”

And voila. Custom loop generators like above is immensely powerful: by extending and/or overriding the default loop generation behavior, and defining custom test sections entirely driven by parameter inputs, users can effectively overload the loop functionality into a dynamic generator of highly abstracted test executor.

Looking for loop definition? Refer to the top of this section.