# Possible to store multiple StructureData

**URL:** <https://aiida.discourse.group/t/possible-to-store-multiple-structuredata/100>\
**Category:** General Usage\
**Tags:** question\
**Created:** [September 12, 2023, 10:18pm UTC](https://aiida.discourse.group/t/possible-to-store-multiple-structuredata/100 "2023-09-12T22:18:52Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![nkeilbart](https://yyz2.discourse-cdn.com/free1/user_avatar/aiida.discourse.group/nkeilbart/32/40_2.png) [@nkeilbart](https://aiida.discourse.group/u/nkeilbart)\
**Post date:** [September 12, 2023, 10:18pm UTC](https://aiida.discourse.group/t/possible-to-store-multiple-structuredata/100/1 "2023-09-12T22:18:52Z")

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Hello everyone,

I’m working on a new workflow within the aiida-environ module. We want to pass in multiple structures and we aren’t sure how many there will be. My initial thought is to make a list of StructureData objects and then store them as a List. This appears to not be allowed. I see that there is a TrajectoryData object but that species that the number of atoms and species must stay constant. What is the best way to store multiple StructureData objects? I wasn’t quite sure how to look that up in the documentation. Thanks.

Nathan

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**Author:** ![sphuber](https://yyz2.discourse-cdn.com/free1/user_avatar/aiida.discourse.group/sphuber/32/6_2.png) [@sphuber](https://aiida.discourse.group/u/sphuber)\
**Post date:** [September 13, 2023, 6:01am UTC](https://aiida.discourse.group/t/possible-to-store-multiple-structuredata/100/2 "2023-09-13T06:01:51Z")

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When you say “you want to pass in”, I take it you mean you want to pass multiple `StructureData` nodes as input to some kind of a process, e.g. a `calcfunction` or `WorkChain`? These have each a separate solutions.

For process functions (such as a `calcfunction`) you can use [variadic arguments](https://aiida.readthedocs.io/projects/aiida-core/en/latest/topics/processes/functions.html#variadic-arguments) just as in normal Python functions:

```python
from aiida.engine import calcfunction

@calcfunction
def take_multiple_structures(**kwargs):
    for link_label, structure in kwargs.items():
        # Here `structure` will be a `StructureData`

# Now you can call it with an arbitrary number of structures. Note that the 
# keys need to be valid identifiers, so just alphanumeric with underscores.
take_multiple_structures(struc_a=StructureData(), struc_b=StructureData(), etc....)

```

For `WorkChains` you would use [dynamic input namespaces](https://aiida.readthedocs.io/projects/aiida-core/en/latest/topics/processes/usage.html#dynamic-namespaces). These namespace simply declare a type, but don’t have to specify exactly which labels they expect but can take any number. Example:

```python
from aiida.engine import WorkChain
from aiida.orm import StructureData

class SomeWorkChain(WorkChain)

    @classmethod
    def define(cls, spec):
        super().define(spec)
        spec.input_namespace('structures', valid_type=StructureData)
        spec.outline(cls.do_something)

    def do_something(self):
        for label, structure in self.inputs.structures.items():
            # Here label is again link label and `structure` the `StructureData` passed as inputs

# This would be launched as 
inputs = {
    'structures': {
        'struc_a': StructureData(),
        'struc_b': StructureData(),
    }
    ....
}
submit(SomeWorkChain, **inputs

```

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<div class="post-metadata">

**Author:** ![nkeilbart](https://yyz2.discourse-cdn.com/free1/user_avatar/aiida.discourse.group/nkeilbart/32/40_2.png) [@nkeilbart](https://aiida.discourse.group/u/nkeilbart)\
**Post date:** [September 13, 2023, 1:09pm UTC](https://aiida.discourse.group/t/possible-to-store-multiple-structuredata/100/3 "2023-09-13T13:09:24Z")

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Thanks Sebastiaan. Hope things are going well over there for you. I’ll look at getting this working today.