From pep-0008:
```
Always use a def statement instead of an assignment statement that binds a
lambda expression directly to an identifier.
Yes:
def f(x): return 2*x
No:
f = lambda x: 2*x
The first form means that the name of the resulting function object is
specifically 'f' instead of the generic '<lambda>'. This is more useful for
tracebacks and string representations in general. The use of the assignment
statement eliminates the sole benefit a lambda expression can offer over an
explicit def statement (i.e. that it can be embedded inside a larger expression)
```
This format is intended for storing data for all sids of an asset type,
e.g. equities or futures for a session. bcolz is not used to avoid the overhead
of creating the directories and files for each asset (which numbers around ~8000
for active equities) can be removed since the update is meant to be read at
once, instead of supporting the random access pattern needed by the simulation.
This patch only adds the reader/writer pair, with the management of finding the
paths to delta files and the application of the updates to the bcolz write left
to internal loader code.
Also, the update reader interface is intentionally constrained to the data for
an entire session to allow for an implementation that allows for mid-session updates.
* ENH: Adds support for supplementary asset mappings
- Adds a supplementary_mappings table to the assets.db, to hold point-
in-time mappings of sids to arbitrary categories of values, e.g.
alternative identifiers. This bumps ASSET_DB_VERSION.
- Adds supplementary_map and supplementary_map_by_sid to AssetFinder,
caches of the underlying table that are fully populated on first
access, which map the supplementary values to sids and vice versa,
respectively.
- Adds lookup_by_supplementary_mapping method, which fronts
supplementary_map to query for the asset last known to have held a
value at a given dt.
- Add get_supplementary_field method, which fronts
supplementary_map_by_sid to query for the last known value held by an
asset at a given dt.
For futures that behave like GC, use the latest roll as the back contract when
walking backwards over the window, so that when the front contract is skipped
because it never has more volume between its auto close date and the previous
auto close date, the back contract which did have volume is still used when
making comparisons to construct the chain.
Eddie Hebert