Why results of map() and list comprehension are different? [duplicate]

by

They are different, because the value of i in both the generator expression and the list comp are evaluated lazily, i.e. when the anonymous functions are invoked in f.
By that time, i is bound to the last value if t, which is -1.

So basically, this is what the list comprehension does (likewise for the genexp):

x = []
i = 1 # 1. from t
x.append(lambda: i)
i = -1 # 2. from t
x.append(lambda: i)

Now the lambdas carry around a closure that references i, but i is bound to -1 in both cases, because that is the last value it was assigned to.

If you want to make sure that the lambda receives the current value of i, do

f(*[lambda u=i: u for i in t])

This way, you force the evaluation of i at the time the closure is created.

Edit: There is one difference between generator expressions and list comprehensions: the latter leak the loop variable into the surrounding scope.

More Related Contents:

  1. How do lexical closures work?
  2. How do I create a list of lambdas (in a list comprehension/for loop)?
  3. List comprehension vs generator expression’s weird timeit results?
  4. yield in list comprehensions and generator expressions
  5. Using while in list comprehension or generator expressions
  6. Are list comprehensions syntactic sugar for `list(generator expression)` in Python 3?
  7. how to create a list of lists using list comprehension [closed]
  8. What do lambda function closures capture?
  9. Don’t understand why UnboundLocalError occurs (closure) [duplicate]
  10. List comprehension vs map
  11. How are lambdas useful? [closed]
  12. Python nonlocal statement
  13. List comprehension rebinds names even after scope of comprehension. Is this right?
  14. Creating a dictionary from a csv file?
  15. Double Iteration in List Comprehension
  16. Can you explain closures (as they relate to Python)?
  17. List vs generator comprehension speed with join function [duplicate]
  18. nonlocal keyword in Python 2.x
  19. How can I do assignments in a list comprehension?
  20. How can I use a conditional expression (expression with if and else) in a list comprehension? [duplicate]
  21. How to return a subset of a list that matches a condition [duplicate]
  22. How can a function access its own attributes?
  23. Python generator expression parentheses oddity
  24. Why list comprehension is much faster than numpy for multiplying arrays?
  25. How to set local variable in list comprehension?
  26. remove None value from a list without removing the 0 value
  27. How to remove duplicate items from a list using list comprehension? [duplicate]
  28. sum each value in a list of tuples
  29. How to limit the size of a comprehension?
  30. Why is Python’s list comprehension loop order backwards? [duplicate]

Leave a Comment