An app is a Parsl construct for representing a fragment of Python code or external Bash shell code that can be asynchronously executed.

A Parsl app is defined by annotating a Python function with a decorator: the @python_app decorator for a Python app, the @bash_app decorator for a Bash app, and the @join_app decorator for a Join app.

Python apps encapsulate pure Python code, while Bash apps wrap calls to external applications and scripts, and Join apps allow composition of other apps to form sub-workflows.

Python and Bash apps are documented below. Join apps are documented in a later section (see Join Apps)

Python Apps

The following code snippet shows a Python function double(x: int), which returns double the input value. The @python_app decorator defines the function as a Parsl Python app.

def double(x):
    return x * 2


As a Parsl Python app is executed asynchronously, and potentially remotely, the function cannot assume access to shared program state. For example, it must explicitly import any required modules and cannot refer to variables used outside the function. Thus while the following code fragment is valid Python, it is not valid Parsl, as the bad_double() function requires the random module and refers to the external variable factor.

import random
factor = 5

def bad_double(x):
    return x * random.random() * factor


The following alternative formulation is valid Parsl.

import random
factor = 5

def good_double(x, f):
    import random
    return x * random.random() * f

print(good_double(42, factor))

Python apps may be passed any Python input argument, including primitive types, files, and other complex types that can be serialized (e.g., numpy array, scikit-learn model). They may also be passed a Parsl Future (see Futures) returned by another Parsl app. In this case, Parsl will establish a dependency between the two apps and will not execute the dependent app until all dependent futures are resolved. Further detail is provided in Futures.

A Python app may also act upon files. In order to make Parsl aware of these files, they must be specified by using the inputs and/or outputs keyword arguments, as in following code snippet, which copies the contents of one file (in.txt) to another (out.txt).

def echo(inputs=[], outputs=[]):
    with open(inputs[0], 'r') as in_file, open(outputs[0], 'w') as out_file:

echo(inputs=[in.txt], outputs=[out.txt])

Special Keyword Arguments

Any Parsl app (a Python function decorated with the @python_app or @bash_app decorator) can use the following special reserved keyword arguments.

  1. inputs: (list) This keyword argument defines a list of input Futures or files. Parsl will wait for the results of any listed Futures to be resolved before executing the app. The inputs argument is useful both for passing files as arguments and when one wishes to pass in an arbitrary number of futures at call time.

  2. outputs: (list) This keyword argument defines a list of files that will be produced by the app. For each file thus listed, Parsl will create a future, track the file, and ensure that it is correctly created. The future can then be passed to other apps as an input argument.

  3. walltime: (int) This keyword argument places a limit on the app’s runtime in seconds. If the walltime is exceed, Parsl will raise an exception.


A Python app returns an AppFuture (see Futures) as a proxy for the results that will be returned by the app once it is executed. This future can be inspected to obtain task status; and it can be used to wait for the result, and when complete, present the output Python object(s) returned by the app. In case of an error or app failure, the future holds the exception raised by the app.


There are some limitations on the Python functions that can be converted to apps:

  1. Functions should act only on defined input arguments. That is, they should not use script-level or global variables.

  2. Functions must explicitly import any required modules.

  3. Parsl uses cloudpickle and pickle to serialize Python objects to/from apps. Therefore, Parsl require that all input and output objects can be serialized by cloudpickle or pickle. See Addressing SerializationError.

  4. STDOUT and STDERR produced by Python apps remotely are not captured.

Bash Apps

A Parsl Bash app is used to execute an external application, script, or code written in another language. It is defined by a @bash_app decorator and the Python code that forms the body of the function must return a fragment of Bash shell code to be executed by Parsl. The Bash shell code executed by a Bash app can be arbitrarily long.

The following code snippet presents an example of a Bash app echo_hello, which returns the bash command 'echo "Hello World!"' as a string. This string will be executed by Parsl as a Bash command.

def echo_hello(stderr='std.err', stdout='std.out'):
    return 'echo "Hello World!"'

# echo_hello() when called will execute the shell command and
# create a std.out file with the contents "Hello World!"

Unlike a Python app, a Bash app cannot return Python objects. Instead, Bash apps communicate with other apps via files. A decorated @bash_app exposes the inputs and outputs keyword arguments described above for tracking input and output files. It also includes, as described below, keyword arguments for capturing the STDOUT and STDERR streams and recording them in files that are managed by Parsl.

Special Keywords

In addition to the inputs, outputs, and walltime keyword arguments described above, a Bash app can accept the following keywords:

  1. stdout: (string, tuple or parsl.AUTO_LOGNAME) The path to a file to which standard output should be redirected. If set to parsl.AUTO_LOGNAME, the log will be automatically named according to task id and saved under task_logs in the run directory. If set to a tuple (filename, mode), standard output will be redirected to the named file, opened with the specified mode as used by the Python open function.

  2. stderr: (string or parsl.AUTO_LOGNAME) Like stdout, but for the standard error stream.

  3. label: (string) If the app is invoked with stdout=parsl.AUTO_LOGNAME or stderr=parsl.AUTO_LOGNAME, this arugment will be appended to the log name.

A Bash app can construct the Bash command string to be executed from arguments passed to the decorated function.

def echo(arg, inputs=[], stderr=parsl.AUTO_LOGNAME, stdout=parsl.AUTO_LOGNAME):
    return 'echo {} {} {}'.format(arg, inputs[0], inputs[1])

future = echo('Hello', inputs=['World', '!'])
future.result() # block until task has completed

with open(future.stdout, 'r') as f:
    print( # prints "Hello World !"


A Bash app, like a Python app, returns an AppFuture, which can be used to obtain task status, determine when the app has completed (e.g., via future.result() as in the preceding code fragment), and access exceptions. As a Bash app can only return results via files specified via outputs, stderr, or stdout; the value returned by the AppFuture has no meaning.

If the Bash app exits with Unix exit code 0, then the AppFuture will complete. If the Bash app exits with any other code, Parsl will treat this as a failure, and the AppFuture will instead contain an BashExitFailure exception. The Unix exit code can be accessed through the exitcode attribute of that BashExitFailure.


The following limitation applies to Bash apps:

  1. Environment variables are not supported.