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Frequently Asked Questions

1. What's the difference to pynbody?
2. Why does my script take so much memory?
3. Why does the derived block "xy" not appear in snap.derivable_blocks() though defined in derived.cfg?
4. Why does the block "xy" not appear in snap.always_cached_derived though listed for always_cache in derived.cfg?
5. I get a UnitError that two units such as 'ckpc/h_0' and 'kpc' are not convertible. Is this a bug?

Answers

1. What's the difference to pynbody?

Indeed, pygad and pynbody are very similar. pygad is more light-weight and especially designed for Gadget, though.

2. Why does my script take so much memory?

Well, pygad is Python and that is not a very efficient language -- but nice to use one! A pure C(++) would most likely be more efficient, however, there are some things you can do:

• Call pygad.gc_full_collect(), specially when looping over snapshots (in this case at the end/beginning of each loop): this will more regularly free "unreachable memory" (see garbage collector).
• Stop pygad from caching derived blocks: for a single snapshot by setting snap.cache_derived = False, or globally by setting cache_derived = False in derived.cfg or pygad.snapshot.derived.general['cache_derived'] = False.

Both will make your script a little slower though. However, that's mostly insignificant.

For some fine-tuning, you can adjust the list of always cached derived blocks (always_cache in derived.cfg) and/or explicitly free blocks that are no longer needed by calling del snap[<block_name>] in you script.

3. Why does the derived block "xy" not appear in snap.derivable_blocks() though defined in derived.cfg?

This can be due to different reasons:

• Not derivable due to missing required blocks. If "xy" depends on some other block "ab" that is neither loadable or derivable, "xy" will not be listed in derivable_blocks() even if properly defined.
• This block is actually loadable, which is why the derived block is simply ignored. Loadable blocks have priority over derived blocks.
• There is an actual bug...

4. Why does the block "xy" not appear in snap.always_cached_derived though listed for always_cache in derived.cfg?

The block is most likely either not defined or not derivable (see also question 3). Otherwise its probably a bug. Please, report!

5. I get a UnitError that two units such as 'ckpc/h_0' and 'kpc' are not convertible. Is this a bug?

No, that is (most likely) no bug! To convert the units given here the scale parameter 'a' and the Hubble parameter 'h_0' are needed. They can be specified by the subs argument for convert_to/in_units_of by either a Python dictionary or a snapshot (see the documentation!). The latter has values for those and pygad is using them.

In case of SimArrs there is a associated snapshot from which these cosmological parameters are automatically taken from. This is why in some cases (namely those where SimArrs are used) you don't need to specify them.