PyYAML multi-line strings

Hello,

We (Pyrseas project) are trying to deal with text returned by PostgreSQL
representing either SQL views or functions (stored procedures).  Until
now, the text was retrieved from the database into a Python string and
then passed to yaml.dump.  The output usually ended up as quoted or
unquoted strings, e.g.,

# function
    source: SELECT $1 + $1;
# view
    definition: SELECT DISTINCT product.product_id,
product.product_code, product.product_description
      FROM (warehouse.inventory JOIN product.product USING (product_id));
# or
    definition: " SELECT DISTINCT product.product_id,
product.product_code, product.product_description\n\
      \   FROM warehouse.inventory\n   JOIN product.product USING
(product_id);"

The first view was fetched in "raw" format, i.e., as a single line, and
apparently the line break before the "FROM" is just a fortunate
coincidence.  The second view was requested "prettified" so the actual
value in Python looks like:

def = """SELECT DISTINCT product.product_id, product.product_code,
product.product_description
   FROM warehouse.inventory
   JOIN product.product USING (product_id);"""

A contributor suggested using the following to get the output formatted
properly:

--- test.py ---
import yaml


class MultiLineStr(str):
    """ Marker for multiline strings"""


def MultiLineStr_presenter(dumper, data):
    return dumper.represent_scalar('tag:yaml.org,2002:str', data, style='|')
yaml.add_representer(MultiLineStr, MultiLineStr_presenter)
--- (continues below) ---

The view definitions and function source strings were then changed to be
instances of MultiLineStr.  This resulted in the following output (using
the prettified views):

# function
    source: |-
      SELECT $1 * $1;
# view
    definition: |2-
       SELECT DISTINCT product.product_id, product.product_code,
product.product_description
         FROM warehouse.inventory
         JOIN product.product USING (product_id);

However, in some cases, PyYAML still outputs the quoted string with
embedded newlines.  I've created a simple test case, see below.  In the
second definition, I simply copied the first but removed the embedded
\n's.  That causes it to be shown with the block scalar indicator,
albeit as a single line.  That made me believe that the newlines were
causing the problem, but the third definition outputs correctly even
though it has newlines.  And I also have a longer text definition (not
shown) so the text length isn't the cause either.

I would appreciate any help or guidance with the above.  Thanks in advance.

Regards,


Joe


--- test.py (continued) ---

views = [
    {'definition': ' SELECT cu.customer_id AS id, (cu.first_name::text
|| \' \'::text) || cu.last_name::text AS name, a.address, a.postal_code
AS "zip code", a.phone, city.city, country.country, \n        CASE\n
        WHEN cu.activebool THEN \'active\'::text\n            ELSE
\'\'::text\n        END AS notes, cu.store_id AS sid\n   FROM customer
cu\n   JOIN address a ON cu.address_id = a.address_id\n   JOIN city ON
a.city_id = city.city_id\n   JOIN country ON city.country_id =
country.country_id;',
     },
    {'definition': ' SELECT cu.customer_id AS id, (cu.first_name::text
|| \' \'::text) || cu.last_name::text AS name, a.address, a.postal_code
AS "zip code", a.phone, city.city, country.country,         CASE
    WHEN cu.activebool THEN \'active\'::text            ELSE \'\'::text
       END AS notes, cu.store_id AS sid   FROM customer cu   JOIN
address a ON cu.address_id = a.address_id   JOIN city ON a.city_id =
city.city_id   JOIN country ON city.country_id = country.country_id;',
     },
    {'definition': """ SELECT e.month, e.day, e.year, e.topic_id,
t.topic_name
    FROM event e
    JOIN topic t USING (topic_id)
   WHERE e.type = 123;"""
     }]

for view in views:
    print(yaml.dump(view, default_flow_style=False))
for view in views:
    view['definition'] = MultiLineStr(view['definition'])
    print(yaml.dump(view, default_flow_style=False))
--- end ---

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