Summary

When using the API, repeated calls to Table.convert() do not work correctly since all conversions quietly use the callable (function, lambda) from the first call to convert() only. Subsequent invocations with different callables use the callable from the first invocation only.

Example

from sqlite_utils import Database

db = Database(memory=True)
table = db['table']
col = 'x'
table.insert_all([{col: 1}])
print(table.get(1))

table.convert(col, lambda x: x*2)
print(table.get(1))

def zeroize(x):
    return 0
#zeroize = lambda x: 0
#zeroize.__name__ = 'zeroize'
table.convert(col, zeroize)
print(table.get(1))

Output:

{'x': 1}
{'x': 2}
{'x': 4}

Expected:

{'x': 1}
{'x': 2}
{'x': 0}

Explanation

This is some relevant documentation.

• Table.convert() takes a Callable to perform data conversion on a column
• The Callable is passed to Database.register_function()
• Database.register_function() uses the callable's __name__ attribute for registration
• (Aside: all lambdas have a __name__ of <lambda>: I thought this was the problem, and it was close, but not quite)
• However convert() first wraps the callable by local function convert_value()
• Consequently register_function() sees name convert_value for all invocations from convert()
• register_function() silently ignores registrations using the same name, retaining only the first such registration

There's a mismatch between the comments and the code:

but actually the existing function is returned/used instead (as the "registering custom sql functions" doc I linked above says too). Seems like this can be rectified to match the comment?

Suggested fix

I think there are four things:

1. The call to register_function() from convert()should have an explicit name= parameter (to continue using convert_value() and the progress bar).
2. For functions, this name can be the real function name. (I understand the sqlite api needs a name, and it's nice if those are recognizable names where possible). For lambdas would 'lambda-{uuid}' or similar be acceptable?
3. register_function() really should throw an error on repeated attempts to register a duplicate (function, arity)-pair.
4. A test? I haven't looked at the test framework here but seems this should be testable.

See also

• Support custom names for registered functions #458