range() in Python 2 vs Python 3

In Python 2:

• range(n) eagerly built a list of integers (memory heavy for large n).
• xrange(n) returned an xrange object that generated values lazily.

In Python 3:

• range(n) returns a lazy range object (immutable sequence) that computes values on demand.
• There is no xrange; Py3 range covers that role.
• You can index, slice, and take len(), but it is not a list. Use list(range(...)) if you need a real list.

# Python 3
r = range(1_000_000)
print(type(r))      # <class 'range'>
print(len(r))       # 1000000
print(r[10])        # 10
print(r[:3])        # range(0, 3)
print(list(r[:3]))  # [0, 1, 2]

# Memory: range object is tiny vs equivalent list
import sys
print(sys.getsizeof(range(1_000_000)))
print(sys.getsizeof(list(range(1_000))))  # already much larger for smaller n

Migration note: Code that depended on range returning a list in Py2 must wrap with list(...) under Py3 if mutation/list methods are required.