List comprehension rebinds names even after scope of comprehension. Is this right?

by

List comprehensions leak the loop control variable in Python 2 but not in Python 3. Here’s Guido van Rossum (creator of Python) explaining the history behind this:

We also made another change in Python
3, to improve equivalence between list
comprehensions and generator
expressions. In Python 2, the list
comprehension “leaks” the loop control
variable into the surrounding scope:

x = 'before'
a = [x for x in 1, 2, 3]
print x # this prints '3', not 'before'

This was an artifact of the original
implementation of list comprehensions;
it was one of Python’s “dirty little
secrets” for years. It started out as
an intentional compromise to make list
comprehensions blindingly fast, and
while it was not a common pitfall for
beginners, it definitely stung people
occasionally. For generator
expressions we could not do this.
Generator expressions are implemented
using generators, whose execution
requires a separate execution frame.
Thus, generator expressions
(especially if they iterate over a
short sequence) were less efficient
than list comprehensions.

However, in Python 3, we decided to
fix the “dirty little secret” of list
comprehensions by using the same
implementation strategy as for
generator expressions. Thus, in Python
3, the above example (after
modification to use print(x) 🙂 will
print ‘before’, proving that the ‘x’
in the list comprehension temporarily
shadows but does not override the ‘x’
in the surrounding scope.

More Related Contents:

  1. Find a unique element in a list [closed]
  2. What does “list comprehension” and similar mean? How does it work and how can I use it?
  3. Flattening a shallow list in Python [duplicate]
  4. Is it Pythonic to use list comprehensions for just side effects?
  5. List comprehension without [ ] in Python
  6. Generator expressions vs. list comprehensions
  7. Are list-comprehensions and functional functions faster than “for loops”?
  8. List comprehension on a nested list?
  9. Create list of single item repeated N times
  10. if else in a list comprehension [duplicate]
  11. Why is there no tuple comprehension in Python?
  12. List comprehension with if statement
  13. Transpose a matrix in Python [duplicate]
  14. recursive iteration through nested json for specific key in python
  15. How to unzip a list of tuples into individual lists? [duplicate]
  16. Is it possible to use ‘else’ in a list comprehension? [duplicate]
  17. Advanced Nested List Comprehension Syntax
  18. Add an element in each dictionary of a list (list comprehension)
  19. Is it possible to access current object while doing list/dict comprehension in Python?
  20. Python using enumerate inside list comprehension
  21. Nested dictionary comprehension python
  22. How to remove multiple items from a list in just one statement?
  23. How to read aloud Python List Comprehensions?
  24. List with duplicated values and suffix
  25. Flattening a list of NumPy arrays?
  26. How do I convert a tuple of tuples to a one-dimensional list using list comprehension? [duplicate]
  27. flatten list of list through list comprehension
  28. Converting List Comprehensions to For Loops in Python
  29. Skip elements on a condition based in a list comprehension in python
  30. Python list comprehension expensive

Leave a Comment