id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,pull_request,body,repo,type,active_lock_reason,performed_via_github_app,reactions,draft,state_reason
1575131737,I_kwDOCGYnMM5d4ppZ,525,Repeated calls to `Table.convert()` fail,167893,closed,0,,,4,2023-02-07T22:40:47Z,2023-05-08T21:59:41Z,2023-05-08T21:54:02Z,CONTRIBUTOR,,"## 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
```python
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](https://github.com/simonw/sqlite-utils/blob/1491b66dd7439dd87cd5cd4c4684f46eb3c5751b/docs/python-api.rst#registering-custom-sql-functions:~:text=By%20default%20registering%20a%20function%20with%20the%20same%20name%20and%20number%20of%20arguments%20will%20have%20no%20effect).

 * `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()`](https://github.com/simonw/sqlite-utils/blob/fc221f9b62ed8624b1d2098e564f525c84497969/sqlite_utils/db.py#L2661)
 * 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: https://github.com/simonw/sqlite-utils/blob/fc221f9b62ed8624b1d2098e564f525c84497969/sqlite_utils/db.py#L404

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 
- #458 ",140912432,issue,,,"{""url"": ""https://api.github.com/repos/simonw/sqlite-utils/issues/525/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,completed