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How to use function closure to capture common arguments in python

Sometimes you need a function that already 'captures' some information you don't want to repeat when calling it. For example, logging may require you to pass in a lot of contextual information for each call. Instead you can capture the common/unchanging contextual information in a function (using its clsoosure) then call the resulting function with less args (only those that are changing in each call site)

 

class AClass:

  def __init__(self):

    self.make_log_msg = lambda m: f"a={self.a}, b={self.b}, message={m}"


 def do_something(self):

    doStuff();

    logger.info(self.make_log_msg('test message'))


In the absence of the make_log_msg() function you would need to pass all of a,b,c into every logging function call as in self.make_log_msg(self.a, self.b, message). 

This is possible because make_log_msg returns a function (lambda function in this case) that already captures the values self.a and self.b in its closure at the time of creation. Note that the values of self.a and self.b are captured at the time of the function assignment.


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