render_python_code crashing when using server_default option in Column

[ElPincheTopo]

The function alembic.autogenerate.render_python_code doesn't work when you use server_default=func.current_timestamp()) in a column.

It used to work in alembic<=1.0.10 but it seems to be broken in any version since 1.0.11 up to 1.3.1. I'm using python 3.8, postgres 11.4 on ubuntu 18.10.

This is how to reproduce the problem:

from sqlalchemy import Table, Column, DateTime, func, MetaData, create_engine
from alembic.autogenerate import produce_migrations, render_python_code
from alembic.runtime.migration import MigrationContext

meta = MetaData()
engine = create_engine('postgresql+psycopg2://postgres:@localhost:32788/postgres')

c = Column('created_at', DateTime, server_default=func.current_timestamp())
t = Table('some_table', meta, c)

mc = MigrationContext.configure(engine.connect(), opts={'compare_type': True})
mig = produce_migrations(mc, meta)
python_code = render_python_code(mig.upgrade_ops, render_as_batch=False)

print(python_code)

This is the output on alembic <= 1.0.10:

# ### commands auto generated by Alembic - please adjust! ###
    op.create_table('some_table',
    sa.Column('created_at', sa.DateTime(), server_default=sa.text('CURRENT_TIMESTAMP'), nullable=True)
    )
# ### end Alembic commands ###

And in alembic >= 1.0.11 it fails on this line:

python_code = render_python_code(mig.upgrade_ops, render_as_batch=False)

This is the output:

  File ".virtualenv/lib/python3.8/site-packages/alembic/autogenerate/api.py", line 164, in render_python_code
    render._render_cmd_body(up_or_down_op, autogen_context)
  File ".virtualenv/lib/python3.8/site-packages/alembic/autogenerate/render.py", line 71, in _render_cmd_body
    lines = render_op(autogen_context, op)
  File ".virtualenv/lib/python3.8/site-packages/alembic/autogenerate/render.py", line 87, in render_op
    lines = util.to_list(renderer(autogen_context, op))
  File ".virtualenv/lib/python3.8/site-packages/alembic/autogenerate/render.py", line 172, in _add_table
    for col in [
  File ".virtualenv/lib/python3.8/site-packages/alembic/autogenerate/render.py", line 173, in <listcomp>
    _render_column(col, autogen_context) for col in table.columns
  File ".virtualenv/lib/python3.8/site-packages/alembic/autogenerate/render.py", line 601, in _render_column
    rendered = _render_server_default(
  File ".virtualenv/lib/python3.8/site-packages/alembic/autogenerate/render.py", line 650, in _render_server_default
    return _render_potential_expr(
  File ".virtualenv/lib/python3.8/site-packages/alembic/autogenerate/render.py", line 509, in _render_potential_expr
    "sql": autogen_context.migration_context.impl.render_ddl_sql_expr(
AttributeError: 'NoneType' object has no attribute 'impl'

Not sure if I'm doing something wrong or if this was accidentally broken in release 1.0.11.

[zzzeek]

regression caused by #579

[sqla-tester]

Mike Bayer has proposed a fix for this issue in the master branch:

Provide a context for render_python_code https://gerrit.sqlalchemy.org/1610

[zzzeek]

this will be in the next release. there is no workaround other than using the private APIs directly and passing in your migration context, as in this patch:

diff --git a/alembic/autogenerate/api.py b/alembic/autogenerate/api.py
index 709e97d..db929d0 100644
--- a/alembic/autogenerate/api.py
+++ b/alembic/autogenerate/api.py
@@ -143,6 +143,7 @@ def render_python_code(
     render_as_batch=False,
     imports=(),
     render_item=None,
+    migration_context=None,
 ):
     """Render Python code given an :class:`.UpgradeOps` or
     :class:`.DowngradeOps` object.
@@ -158,7 +159,15 @@ def render_python_code(
         "render_as_batch": render_as_batch,
     }
 
-    autogen_context = AutogenContext(None, opts=opts)
+    if migration_context is None:
+        from ..runtime.migration import MigrationContext
+        from sqlalchemy.engine.default import DefaultDialect
+
+        migration_context = MigrationContext.configure(
+            dialect=DefaultDialect()
+        )
+
+    autogen_context = AutogenContext(migration_context, opts=opts)
     autogen_context.imports = set(imports)
     return render._indent(
         render._render_cmd_body(up_or_down_op, autogen_context)