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add notes on using libDetDescr with numpy #1253

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Jan 31, 2024
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add notes on using libDetDescr with numpy #1253

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add notes on using libDetDescr with numpy
tomeichlersmith Jan 23, 2024
d84ba68
add link to numpy docs describing what this is doing
tomeichlersmith Jan 23, 2024
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25 changes: 22 additions & 3 deletions DetDescr/README.md
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Original file line number Diff line number Diff line change
@@ -1,7 +1,7 @@
# Accessing DetectorIDs from Python

<p align="center">
<img src="https://github.com/LDMX-Software/ldmx-software.github.io/blob/trunk/img/ldmx_logo_dark.png" width="500">
<img src="https://github.com/LDMX-Software/ldmx-software.github.io/blob/trunk/src/img/ldmx_logo_dark.png" width="500">
</p>

## Building DetDescr with DetectorID binding support
Expand Down Expand Up @@ -50,9 +50,28 @@ HcalDigiID [raw, section, layer, strip, end] (411042050, 0, 1, 2, 0)
->HcalID [raw, section, layer, strip] (402654210, 0, 1, 2)
```

## Getting documentation from within a Python session
``` python
### Usage with `numpy`
Many python-based analyses use `numpy` or `numpy`-like libraries (e.g. `pandas` and `awkward`) which
"vectorize" the operation of applying the same function to all of the elements of an array. We can do
the same thing here in order to leverage the translation of raw ID numbers (which are stored in the data files)
into different detector information. Below is an example of using this class to extract the layer index
for each raw id.
```python
from libDetDescr import EcalID
import numpy as np
@np.vectorize
def to_layer(rawid):
return EcalID(int(rawid)).layer()
# id is a np.array of raw ID numbers e.g. read in from the EcalSimHits_test.id_ branch of LDMX_Events
layer = to_layer(id)
```
This isn't very performant on larger arrays; however, it does allow the user to avoid a raw python `for` loop.
Effectively, we are using a [non-ufunc extension](https://numpy.org/doc/stable/user/c-info.ufunc-tutorial.html#example-non-ufunc-extension).
We could look at incorporating numpy into our Python bindings
(via [Boost.Python NumPy](https://live.boost.org/doc/libs/1_65_1/libs/python/doc/html/numpy/index.html))
to make this as performant as possible.

## Getting documentation from within a Python session

Boost.Python will automatically generate some documentation for each kind of
DetectorID that you can access through the built-in help system in Python.
Expand Down
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