Pass-by-reference means that your callee gets a reference to your local variables, and can modify them. This is impossible in Python. Pass by value means that your callee gets the values of your local variables and can't modify them. This is how Python functions work.

What those values represent and how they can be used is a completely different topic. Take the following code:

  x = "/dirs/sub/file.txt"
  with open(x, "w") as file:
    file.write("abc")
  foo(x)
  with open(x, "r") as file:
    print(file.read_all()) #prints "def" 

  def foo(z):
    with open(z, "w") as file:
      file.write("def")
      

Here x is in essence a "reference to a file". When you pass x to foo, it gets a copy of that reference in z. But both x and z refer to the same file, so when you modify the file, both see the changes. The calling convention is passing a copy of the value to the function. It doesn't care what that value represents.

So to be very clear:

  def foo(x):
    x['a'] = 1
  
  y = {'b': 2}
  foo(y)
  print(y)

foo can modify the object y points to, but it can't make y point to a different object? Is that what "This is impossible in Python" is referring to?

Yes.