In Python what does dict pop a b mean

Question

class a object       data   a   aaa   b   bbb   c   ccc       def pop self  key   args               return self data pop key   args  what is this mean   b a   print b pop  a    b   bbb    print b data   self data pop key   args   larr ------ why is there a second argument

User · Accepted Answer

The pop method of dicts  like self data  i e    a   aaa   b   bbb   c   ccc    here  takes two arguments -- see the docs  The second argument  default  is what pop returns if the first argument  key  is absent   If you call pop with just one argument  key  it raises an exception if that key s absent    In your example  print b pop  a    b   bbb     this is irrelevant because  a  is a key in b data  But if you repeat that line      b a   print b pop  a    b   bbb    print b pop  a    b   bbb    print b data   you ll see it makes a difference  the first pop removes the  a  key  so in the second pop the default argument is actually returned  since  a  is now absent from b data

User · Answer

So many questions here  I see at least two  maybe three    What does pop a b  do  Why are there a second argument  What is  args being used for      The first question is trivially answered in the Python Standard Library reference      pop key   default        If key is in the dictionary  remove it and return its value  else return default    If default is not given and key is not in the dictionary  a KeyError is raised      The second question is covered in the Python Language Reference      If the form     identifier    is present    it is initialized to a tuple receiving   any excess positional parameters    defaulting to the empty tuple  If the   form      identifier    is present  it is   initialized to a new dictionary   receiving any excess keyword   arguments  defaulting to a new empty   dictionary    In other words  the pop function takes at least two arguments  The first two get assigned the names self and key  and the rest are stuffed into a tuple called args   What s happening on the next line when  args is passed along in the call to self data pop is the inverse of this - the tuple  args is expanded to of positional parameters which get passed along  This is explained in the Python Language Reference      If the syntax  expression appears in   the function call  expression must   evaluate to a sequence  Elements from   this sequence are treated as if they   were additional positional arguments   In short  a pop   wants to be flexible and accept any number of positional parameters  so that it can pass this unknown number of positional parameters on to self data pop     This gives you flexibility  data happens to be a dict right now  and so self data pop   takes either one or two parameters  but if you changed data to be a type which took 19 parameters for a call to self data pop   you wouldn t have to change class a at all  You d still have to change any code that called a pop   to pass the required 19 parameters though

User · Answer

def func  args        pass   When you define a function this way   args will be array of arguments passed to the function  This allows your function to work without knowing ahead of time how many arguments are going to be passed to it    You do this with keyword arguments too  using   kwargs   def func2   kwargs        pass   See  Arbitrary argument lists    In your case  you ve defined a class which is acting like a dictionary  The dict pop method is defined as pop key   default      Your method doesn t use the default parameter  But  by defining your method with  args and passing  args to dict pop    you are allowing the caller to use the default parameter   In other words  you should be able to use your class s pop method like dict pop    my a   a   value1   my a pop  key1           throw an exception if key1 isn t in the dict value2   my a pop  key2   None    return None if key2 isn t in the dict

User · Answer

gt  gt  gt  def func a   args    kwargs         print  a  s  args  s  kwargs  s     a  args  kwargs        gt  gt  gt  func  one    two    three   four  four   five  five   a one  args   two    three    kwargs   four    four    five    five        gt  gt  gt  def anotherfunct beta   args         print  beta  s  args  s     beta  args        gt  gt  gt  def func a   args    kwargs         anotherfunct a   args        gt  gt  gt  func  one    two    three   four  four   five  five   beta one  args   two    three    gt  gt  gt

[python] In Python, what does dict.pop(a,b) mean?

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