Add a parameter into kwargs during function call?

Question

Is there a way to add key-value-pair into kwargs during the function call?

def f(**kwargs):
    print(kwargs)

# ...

pre_defined_kwargs = {'a': 1, 'b': 2}

f(**pre_defined_kwargs, c=3)

Or even change the existing arguments?

f(**pre_defined_kwargs, b=3)  # replaces the earlier b=2

These two examples don't work, as they raise error

>>> f(**pre_defined_kwargs, c=3)
SyntaxError: invalid syntax

Pointing at the comma in between the arguments

Answer 1

For Python versions < 3.5, you need to place the **kwargs variable keyword argument last:

f(c=3, **pre_defined_kwargs)

See the Calls expression syntax; in all forms of the grammar the "**" expression rule is placed last. In other words, using anything after the **expression syntax is a syntax error.

If you want to update the dictionary with new values, you can use the dict() callable; it can create a copy of your existing dictionary and update keys in that, provided the keys are also valid Python identifiers (start with a letter or underscore, and only contain letters, digits and underscores):

f(c=3, **dict(pre_defined_kwargs, b=42))

Here b=42 sets a new value for the 'b' key. This same syntax can be used to add keys too, of course.

You cannot use the same key both in the **expression mapping and explicitly; that'll raise a TypeError. Again, from the documentation already linked:

If the syntax **expression appears in the function call, expression must evaluate to a mapping, the contents of which are treated as additional keyword arguments. In the case of a keyword appearing in both expression and as an explicit keyword argument, a TypeError exception is raised.

Demo:

>>> def f(**kwargs):
...     print(kwargs)
... 
>>> pre_defined_kwargs = {'a': 1, 'b': 2}
>>> f(c=3, **pre_defined_kwargs)
{'a': 1, 'c': 3, 'b': 2}
>>> dict(pre_defined_kwargs, b=42)
{'a': 1, 'b': 42}
>>> f(c=3, **dict(pre_defined_kwargs, b=42))
{'a': 1, 'c': 3, 'b': 42}

This restriction has been lifted as of Python 3.5 (thanks to PEP-448 -- Additional Unpacking Generalizations; you can now freely mix the order of argument types and use multiple **mapping references in a call (using distinct mappings). Keywords still have to be unique across all arguments applied; you still can't 'override' arguments that appear more than once.