Test Your Knowledge: Answers

 

 
  1. List comprehensions in square brackets produce the result list all at once in memory. When they are enclosed in parentheses instead, they are actually generator expressions—they have a similar meaning but do not produce the result list all at once. Instead, generator expressions return a generator object, which yields one item in the result at a time when used in an iteration context.
  2. Generators are objects that support the iteration protocol—they have a __next__ method that repeatedly advances to the next item in a series of results and raises an exception at the end of the series. In Python, we can code generator functions with def, generator expressions with parenthesized list comprehensions, and generator objects with classes that define a special method named __iter__ (discussed later in the book).
  3. A generator function has a yield statement somewhere in its code. Generator functions are otherwise identical to normal functions syntactically, but they are compiled specially by Python so as to return an iterable object when called.
  4. When present, this statement makes Python compile the function specially as a generator; when called, the function returns a generator object that supports the iteration protocol. When the yield statement is run, it sends a result back to the caller and suspends the function’s state; the function can then be resumed after the last yield statement, in response to a next built-in or __next__ method call issued by the caller. Generator functions may also have a return statement, which terminates the generator.
  5. The map call is similar to a list comprehension—both build a new list by collecting the results of applying an operation to each item in a sequence or other iterable, one item at a time. The main difference is that map applies a function call to each item, and list comprehensions apply arbitrary expressions. Because of this, list comprehensions are more general; they can apply a function call expression like map, but map requires a function to apply other kinds of expressions. List comprehensions also support extended syntax such as nested for loops and if clauses that subsume the filter built-in.