الدرس 09 · Collections and functions

Comprehensions

لم يُترجم بعد؛ يُعرض باللغة الإنجليزية

A list comprehension builds a new list from an old one in a single expression: square brackets around an expression, a for clause and an optional if clause. Step through big = [n * 10 for n in nums if n > 2] one item at a time: watch for take each value, if test it, and the expression turn the ones that pass into new items, so the new list grows to [40, 50]. Then see the same work written as a plain for loop.

12 دقيقة10 خطوات5 أسئلةBeginner

شكل 9 · تفاعلي

جارٍ تحميل الرسم…

أو ← →

اختبر نفسك

5 أسئلة سريعة

اختر إجابة لتعرف السبب. تُحفَظ نتيجتك مع تقدّمك.

01What list does [n * 2 for n in [1, 2, 3]] build?

The for clause takes 1, 2 and 3 in turn, and the expression n * 2 is worked out for each one: [2, 4, 6].

02What does [n for n in [5, 1, 8, 3] if n > 4] give?

The if clause keeps only the items whose test is True. 5 > 4 and 8 > 4 are True; 1 and 3 are dropped, and the order of the original list is kept.

03For each item, in what order does Python run the parts of [n * 10 for n in nums if n > 2] ?

The expression is written first but runs last: for gives the item a name, if tests it, and only an item that passes reaches the expression.

04After big = [n * 10 for n in nums if n > 2] runs, what is nums?

A comprehension builds a brand-new list and gives it to big. The list it reads from is not changed.

05Which for loop builds the same list as big = [n * 10 for n in nums if n > 2] ?

Start with an empty list, walk through nums, test each item, and append the expression for the ones that pass. One difference: after the loop the name n is still set (to the last item); after the comprehension it is not.

المصطلحات الأساسية

المسرد

List comprehension: A short way to build a new list from another: [expression for name in list if test], inside square brackets.
Expression: The part written first, such as n * 10: what each kept item becomes in the new list.
for clause: for n in nums: takes the items of a list one at a time, left to right, and gives each the name n.
if clause: An optional filter, such as if n > 2: only items whose test is True reach the expression.
Index: An item's position in a list, counting from 0: nums[0] is the first item.
append(): A list method that adds one item to the end of a list; a comprehension does this for every kept item.
Dict and set comprehensions: The same idea in curly braces: {n: n * 10 for n in nums} builds a dict, {n % 2 for n in nums} a set.

تفاصيل إضافية

الملاحظات والمصادر

The comprehension is split over four lines so each part gets its own line; Python joins lines inside brackets, so it is the same as big = [n * 10 for n in nums if n > 2].
For each item Python runs the clauses in the order for, then if, then the expression, even though the expression is written first.
In Python 3 a comprehension runs in its own small scope, so its loop name (n here) does not leak out; a plain for loop does leave n set to the last item.
A list comprehension always builds a brand-new list; the list it reads from (nums) is not changed.
Curly braces make a set comprehension, {n % 2 for n in nums}, or with key: value a dict comprehension, {n: n * 10 for n in nums}.

اختبار يتاح بعد الخطوة الأخيرة.