PyPika control order of with clauses - pypika

Question

I am using PyPika (version 0.37.6) to create queries to be used in BigQuery. I am building up a query that has two WITH clauses, and one clause is dependent on the other. Due to the dynamic nature of my application, I do not have control over the order in which those WITH clauses are added to the query.
Here is example working code:
a_alias = AliasedQuery("a")
b_alias = AliasedQuery("b")
a_subq = Query.select(Term.wrap_constant("1").as_("z")).select(Term.wrap_constant("2").as_("y"))
b_subq = Query.from_(a_alias).select("z")
q = Query.with_(a_subq, "a").from_(a_alias).select(a_alias.y)
q = q.with_(b_subq, "b").from_(b_alias).select(b_alias.z)
sql = q.get_sql(quote_char=None)
That generates a working query:
WITH a AS (SELECT '1' z,'2' y) ,b AS (SELECT a.z FROM a) SELECT a.y,b.z FROM a,b
However, if I add the b WITH clause first, then since a is not yet defined, the resulting query:
WITH b AS (SELECT a.z FROM a), a AS (SELECT '1' z,'2' y) SELECT a.y,b.z FROM a,b
does not work. Since BigQuery does not support WITH RECURSIVE, that is not an option for me.
Is there any way to control the order of the WITH clauses? I see the _with list in the QueryBuilder (the type of variable q), but since that's a private variable, I don't want to rely on that, especially as new versions of PyPika may not operate the same way.
One way I tried to do this is to always insert the first WITH clause at the beginning of the _with list, like this:
q._with.insert(0, q._with.pop())
Although this works, I'd like to use a PyPika supported way to do that.
In a related question, is there a supported way within PyPika to see what has already been added to the select list or other parts of the query? I noticed the q.selects member variable, but selects is not part of the public documentation. Using q.selects did not actually work for me when using our project's Python version (3.6) even though it did work in Python 3.7. The code I was trying to use is:
if any(field.name == "date" for field in q.selects if isinstance(field, Field))
The error I got was as follows:
def __getitem__(self, item: slice) -> "BetweenCriterion":
if not isinstance(item, slice):
> raise TypeError("Field' object is not subscriptable")
Thank you in advance for your help.

Answer 1

I could not figure out how to control the order of the WITH clauses after calling query.with_() (except for the hack already noted). As a result, I restructured my application to get around this problem. I am now calling query.with_() before building up the rest of the query.
This also made my related question moot, because I no longer need to see what I've already added to the query.