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pythonlambda
Med malik
Med malik

Reputation: 1

Lambda event handler might not be declared?

So after reading a while on lambda event handler it really confused me. for example I have these two lines of code :

b1 = Button(root,text="Show",command=(lambda e=ents :fetch(e)))

root.bind("<Return>",(lambda event, e=ents : fetch(e)))

ents is a function that returns a list of tuples. let's say [("a",x),("b",y)] while x and y are obtained from an entry (texbox) just to illustrate my work. fetch is a simple function that prints out the tuples

def fetch(entries): 
    for entry in entries: 
        field = entry[0]
        text = entry[1].get() 
        print ("%s: %s" %(field,text))

For the first line, lambda took no event argument and we directly gave the context (ents) , while we used event during the binding. So why do we use an event in one case and we don't in the other ?

Another question, why do we have to assign the list to a variable ( in this case e ) and can't directly use it that way

root.bind("<Return>",(lambda event,ents: fetch(ents)))

because if so, it returns an error : <lambda>() missing 1 required positional argument: 'ents'

Upvotes: 0

Views: 803

Answers (1)

jasonharper
jasonharper

Reputation: 9607

The command parameter of a button is called with no parameters.

The handler for an event binding is called with one parameter, the event itself.

The e=ents in both lambdas, while technically a parameter, does not require or expect any actual parameter to be passed to the lambda; it's simply an idiomatic way of capturing a value for use in the lambda. You could just as easily written something like lambda: fetch(ents). Two slight differences:

  1. e=ents captures the value of ents at the time the lambda was defined, any subsequent changes to the value are ignored. That may or may not be relevant here.
  2. It's slightly more efficient to have the value looked up once, rather than every time the lambda is called. That's completely irrelevant here, but might be a useful optimization in a case where the lambda will be called millions of times.

Upvotes: 2

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