TIL: Bind arguments to dynamically generated lambdas in python

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This past week I had a really weird bug in my kedro pipeline. For some reason data running through my pipeline was coming out completely made no sense, but if I manually request raw data outside of the pipeline it matched expectations.

NOTE While this story is about a kedro pipeline, it can be applied anywhere closures are put into an iterable.

Debugger to the rescue

After a few days of looking at it off and on, I pinpointed that it was all the way down in the raw layer. Right as data is coming off of the database. For this I already had existing sql files stored and a read_sql function to get the data so I opted to just set up the pipeline to utilize the existing code as much as possible, leaning on the kedro framework a bit less.

I have dynamically created lists of pipeline nodes many times in the past, but typically I take data from kedro input and use it in the lambda. I prefer the simplicity of using lambdas over functools.partial . It typically looks something like this.

# 👍  I do this all the time
from kedro.pipeline import node
from my_generic_project_lib import clean

datasets_to_clean = ['sales', 'production', 'inventory']
nodes = []
for dataset in datasets_to_clean:
   nodes.append(
      node(
         func=lambda x: clean(x)
         inputs = f'raw_{dataset}'
         outputs=f'int_{dataset}'
         tags=['int', dataset]
         name=f'create_int_{dataset}'
      )
   )

What was different this time is that I needed to pass in the name of the dataset to my read_sql function, not the data loaded in the framework.

# ❌ This does not work
from kedro.pipeline import node
from my_generic_project_lib import read_sql

datasets_to_read = ['sales', 'production', 'inventory']
nodes = []
for dataset in datasets_to_clean:
   nodes.append(
      node(
         func=lambda: read_sql(dataset) # 💥 The major issue
         inputs = f'dummy'
         outputs=f'int_{dataset}'
         tags=['int', dataset]
         name=f'create_int_{dataset}'
      )
   )

Seriously

As I am still oblivious to what has happened I pop in a breakpoint() and quickly see that during the first run the dataset passed into read_sql was 'inventory' , in fact, every single one was 'inventory' . The lambda is just using the latest value of dataset from outside and has no local dataset attached to it.

The simple fix

# 👍 Much Better
from kedro.pipeline import node
from my_generic_project_lib import read_sql

datasets_to_read = ['sales', 'production', 'inventory']
nodes = []
for dataset in datasets_to_clean:
   nodes.append(
      node(
         func=lambda dataset=dataset: read_sql(dataset) # dataset is now bound to the lambda ✨
         inputs = f'dummy'
         outputs=f'int_{dataset}'
         tags=['int', dataset]
         name=f'create_int_{dataset}'
      )
   )

Try it yourself

I made a slightly more simple example so that you can try it and play with it yourself, edit it, share it with your friends, laugh at my mistake, whatever you like.