Document how to use a `--convert` function that runs initialization code first

When I have an insert command with transform like this:

cat items.json | jq '.data' | sqlite-utils insert listings.db listings - --convert '
d = enchant.Dict("en_US")
row["is_dictionary_word"] = d.check(row["name"])
'  --import=enchant --ignore

I noticed as the number of rows increases the operation becomes quite slow, likely due to the creation of the d = enchant.Dict("en_US") object for each row. Is there a way to share that instance d between transform function calls, like a shared context?

[simonw]

So now this should solve your problem:

echo '[{"name": "notaword"}, {"name": "word"}]
' | python3 -m sqlite_utils insert listings.db listings - --convert '
import enchant
d = enchant.Dict("en_US")

def convert(row):
    global d
    row["is_dictionary_word"] = d.check(row["name"])
'

[simonw]

I found a fix that makes that global workaround unnecessary:

• #472