polars: add a constant to an existing column

Dataframe:

rng = np.random.default_rng(42)df = pl.DataFrame(    {"nrs": [1, 2, 3, None, 5],"names": ["foo", "ham", "spam", "egg", None],"random": rng.random(5),"A": [True, True, False, False, False],    })

Currently, to add a constant to a column I do:

df = df.with_columns(pl.col('random') + 500.0)

Questions:

1. why does df = df.with_columns(pl.col('random') += 500.0) throw a SyntaxError?

2. various AIs tell me that df['random'] = df['random'] + 500 should also work, but it throws the following error instead:

   TypeError: DataFrame object does not support Series assignment by index

   Use DataFrame.with_columns.

This just goes to show how trustworthy AIs are, today! But why is polars throwing an error? I've been using df['random'] to identify the random column in other parts of my code, and it worked.