Chares and Proxies


An application defines new chare types by subclassing from charm4py.Chare. These can have their own custom attributes and methods. In addition, chare classes have to be registered with the runtime when calling charm.start() (see Charm).

Every chare instance has the following properties:


  • thisProxy:

    If the chare is part of a collection, this is a proxy to the collection to which the element belongs to. Otherwise it is a proxy to the individual chare.

  • thisIndex:

    Index of the chare in the collection to which it belongs to.


  • reduce(self, callback, data=None, reducer=None, section=None):

    Perform a reduction operation by giving this chare’s contribution (see Reductions). If section is None, the reduction is performed across the elements of the primary collection to which the chare belongs to. If section is a section proxy, the reduction is performed across the elements of the section (note that the chare must belong to that section).

    The method must be called by all members to perform a successful reduction. callback will receive the result of the reduction. It can be a Future or the remote method of a chare(s) (specified by proxy.method, where proxy can be any type of proxy, including a proxy to a single element or a whole collection). data is this chare’s contribution to the reduction; reducer is the reducer function to apply (see Reducers).

    It is possible to do “empty” reductions (if no data and reducer are given).

  • allreduce(self, data=None, reducer=None, section=None):

    Same as reduce but the call will return a Future which the caller can use to wait for the result (this means that the result of the reduction is sent to all callers). Can only be called from coroutines.

  • AtSync(self):

    Notify the runtime that this chare is ready for load balancing. If load balancing is enabled, load balancing starts on this PE once all of the chares that use “AtSync” have called this. When you create a chare array you specify if its chares use AtSync or not (see Array). Load balancing starts globally once all of the PEs have started load balancing.

  • migrate(self, toPe):

    Requests migration of the chare to the specified PE. The chare must be migratable.


    This should be called via a proxy so that it goes through the scheduler, for example: proxy.migrate(toPe).

    Also note that it is unusual for applications to have to manually migrate chares. Instead, applications should delegate to the runtime’s load balancing framework.

  • migrated(self):

    This is called after a chare has migrated to a new PE. This method is empty, and applications can redefine it in subclasses.

  • setMigratable(self, migratable):

    Set whether the chare is migratable or not (migratable is a bool). If a chare is not migratable and load balancing is enabled, the load balancing framework will not migrate it. All array chares are migratable by default.

Remote methods

Any user-defined methods of chares can be invoked remotely (via Proxies). Note that methods can also be called locally (using standard Python object method invocation). For example, a chare might invoke one of its own methods by doing self.method(*args) thus bypassing remote method invocation. Note that in this case the method will be called directly and will not go through the runtime or scheduler.

Creating single chares

Typically, chares are created as parts of collections (see Groups and Arrays). You can, however, also create individual chares using the following syntax:

  • Chare(chare_type, args=[], onPE=-1):

    where chare_type is the type of chare you want to create. args is the list of arguments to pass to its constructor. If onPE is -1, the runtime decides on which PE to create it. Otherwise it will create the chare on the specified PE. This call returns a proxy.

    You can create any number of chares (of the same or different types).


    This call is asynchronous: it returns immediately without waiting for the chare to be created. See charm.awaitCreation() for one mechanism to wait for creation.


Proxy classes do not exist a priori. They are generated at runtime using metaprogramming, based on the definition of the chare types that are registered when the runtime is started.

Proxy objects are returned when creating chares or collections, and are also stored in the thisProxy attribute of chares.


A proxy object is lightweight and can be sent to any chare(s) in the system via remote methods.

Their methods can also be sent to other chares to use as callbacks (see example below).

Proxies have the same methods as the chare that they reference. Calling those methods will result in the method being invoked on the chare(s) that the proxy references, regardless of the location of the chare.

The syntax to call a remote method is:

proxy.remoteMethod(*args, **kwargs, awaitable=False, ret=False):

Calls the method of the chare(s) referenced by the proxy. This is a remote method invocation. If the proxy references a collection, a broadcast call is made and the method is invoked on all chares in the collection. Otherwise, the method is called on an individual chare. The call returns immediately and does not wait for the method to be invoked at the remote chare(s).

If awaitable is True, the call returns a Future, which can be used to wait for completion. This also works for broadcast calls (wait for the call to complete on every element).

If ret is True, the call returns a Future, which can be used to wait for the result. This also works for broadcast calls. In this case, the return value will be a list of return values, sorted by element index.

If ret or awaitable are True and the remote method throws an unhandled exception, the exception is propagated to the caller (even if the caller is in another PE). The exception is raised at the caller when it queries the future.

Proxies the refer to collections can be sliced to obtain section proxies (see Sections: Split, Slice and Combine Collections).

All proxies implement __eq__ and __hash__, with correct results between proxies generated locally and those obtained from a remote PE. This allows, for example, checking proxies for equality, using them as dictionary keys or inserting in sets.


from charm4py import charm, Chare, Group

class A(Chare):

    def start(self):
        b_proxy = Chare(B)
        # call work and send one of my methods to use as callback

    def recvResult(self, result):
        print('Result is', result)

class B(Chare):

    def work(self, callback):
        # ... do work ...
        result = ...

def main(args):
    a_proxy = Chare(A)