Slicing and comprehensions

A slice selects a range from a list or string. The start index is included and the stop index is excluded. Leaving out an end uses the rest of the sequence.

minutes = [10, 20, 30, 40]
print(minutes[1:3])
print(minutes[:2])
print(minutes[2:])

[20, 30]
[10, 20]
[30, 40]

Build a list from another list

A list comprehension creates a new list from an iterable. Start with a clear for loop when the transformation is complex; use a comprehension for a short, readable rule.

minutes = [10, 20, 30, 40]
doubled = [value * 2 for value in minutes]
long_sessions = [value for value in minutes if value >= 25]
print(doubled)
print(long_sessions)

[20, 40, 60, 80]
[30, 40]

Why this works

A slice returns a new sequence for the requested positions; it does not remove those items from the original list. The stop position is excluded, as it is in range(). A comprehension loops over source values to build a new list and can include a condition to keep only matching values.

Practice

Slice the first three values from a list of five durations. Build a new list containing only durations of at least 20 minutes.

Try another case

For values = [10, 20, 30, 40], what are values[1:3] and [v for v in values if v > 20]?