How to determine object size?

[gnbl]

Does MicroPython have a method to determine object size like sys.getsizeof(_object_)?
I'd like to compare different data structures for memory efficiency (look up a couple thousand timetable entries, for example search in a tuple vs. dictionary).

[jimmo]

sys.getsizeof is not implemented in MicroPython. Internally, the heap does know the allocated size of a given object, but many objects have multiple layers of allocations (e.g. a dictionary object contains a pointer to its table, which contains pointers to keys and values, etc).

getsizeof works in CPython with runtime support - builtin types must implement this to report their size. This would add a lot of code size, so is unlikely to ever be included in standard MicroPython firmware.

If your goal is to understand how much RAM different representations take up, then using gc.mem_alloc() to compute a difference might be more useful.

def make_timetable():
  ...
  return obj

def compute_alloc_diff():
  gc.collect()
  before = gc.mem_alloc()
  x = make_timetable()
  gc.collect()
  print(gc.mem_alloc() - before)

Note that make_timetable is in a function so that any local variables go out of scope -- only the final object will be still "live" when gc.collect() is called the second time.

Some caveats though:

• Ensure that make_timetable doesn't mutate any global state. e.g. modifying global objects.
• It's a bit hard to account for string data, due to string interning, which are stored in chunks. Interned strings are never freed though, so one way to account for this is to call compute_alloc_diff twice. The second time will not include any string data, and only the actual data structures.

[gnbl]

Thanks again.
It looks like tuples might be read from (and kept in) flash memory since compute_alloc_diff() returns zero.
But a binary search (index of next larger element) seems to work nicely on that data (ordered timestamps), so I'll probably not investigate further (const dict or multi-dimensional list).

[jimmo]

Yes, if you're using frozen code, then tuples are constant objects and stored in ROM/flash.