[Sqlalchemy-tickets] Issue #3644: text() colon-escaping behaviour (zzzeek/sqlalchemy)

[Sebastian B. <iss...@bi...>]

New issue 3644: text() colon-escaping behaviour
https://bitbucket.org/zzzeek/sqlalchemy/issues/3644/text-colon-escaping-behaviour

Sebastian Bank:

Using the postgres double colon shortcut for `CAST(expression AS type)`, e.g.:
```python
import sqlalchemy as sa
engine = sa.create_engine('postgresql://postgres@/')
engine.execute('SELECT * FROM pg_attribute WHERE attrelid = %(tab)s::regclass', tab='pg_class')
```

Trying this with `text`, I think the [docs](http://docs.sqlalchemy.org/en/latest/core/sqlelement.html#sqlalchemy.sql.expression.text) would suggest using `\:`s:
```python
engine.execute(sa.text('SELECT * FROM pg_attribute WHERE attrelid = :tab\:\:regclass'), tab='pg_class')
```
But this raises `ProgrammingError: (psycopg2.ProgrammingError)` (rendered as `SELECT * FROM pg_attribute WHERE attrelid = %(tab)s\::regclass`).

As expected, this also raises:
```python
engine.execute(sa.text('SELECT * FROM pg_attribute WHERE attrelid = :tab::regclass'), tab='pg_class')
```
(rendered as `SELECT * FROM pg_attribute WHERE attrelid = :tab::regclass`)

This finally works (extra space): 
```python
engine.execute(sa.text('SELECT * FROM pg_attribute WHERE attrelid = :tab ::regclass'), tab='pg_class')
```
But is this the intended way?