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List comprehensions

A list comprehension builds a new list from a loop in one expression. Add 'if condition' to filter items, or change x (e.g. x * 2) to transform each item.

Part of: VCE Software Development - U4O1 - development and evaluation — Module 102, QCE Digital Solutions Unit 2 — Data and Programming Techniques — Module 57

Understanding List comprehensions in VCE Software Development U4O1

What it does: This is the basic form [x for x in items]: loop over items and collect each x into a new list. The expression at the start (n.upper()) changes each item before it is added to the new list. A filter condition goes at the END of the comprehension, after the loop part: [m for m in marks if m >= 50]. A comprehension creates a new list. The original is only changed if you assign the result back to the same variable name. The 'if x > 10' part filters items. Only items that pass the condition are included in the new list.

Common mistakes: The keyword 'in' is missing. A list comprehension needs 'for x in xs' to loop over the list, so the line should be hi = [x for x in xs if x > 5]. Round brackets create a generator, not a list. A list comprehension uses square brackets: dbl = [p * 2 for p in prices].

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This module's key terms are also in a printable crossword (Modules 82–114) and word search (Modules 3–114).

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Learning

Duo

Which list comprehension doubles every number in nums?

[n for n in nums * 2][n * 2 for n in nums]

Matches

evens = [x for x in nums where x % 2 == 0]

↓

'where' is not Python. A condition in a list comprehension uses the keyword 'if'.

Applying

Meanings

Term

[x for x in items]

Meaning

A list comprehension builds a new list from a loop in one expression. Add 'if condition' to filter items, or change x (e.g. x * 2) to transform each item.

Practice

prices = [10, 25, 40]

Testing

Gaps

doubled = [x * 2 x in nums]

Check Answer

Spots

2# keep numbers bigger than 5

3hi = [x for x xs if x > 5]

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Quick Questions

1. Which list comprehension doubles every number in nums?

A. [n for n in nums * 2]

B. [n * 2 for n in nums] ✓

The expression before 'for' says what each new item is, so n * 2 goes at the start. Putting * 2 after 'in' would repeat the whole list instead.

2. Which comprehension keeps only the marks that are 50 or more?

A. [m for m in marks if m >= 50] ✓

B. [m if m >= 50 for m in marks]

A filter condition goes at the END of the comprehension, after the loop part: [m for m in marks if m >= 50].

3. True or false: a list comprehension builds a NEW list and leaves the original list unchanged.

A. False

B. True ✓

A comprehension creates a new list. The original is only changed if you assign the result back to the same variable name.

4. Which line is the correct list comprehension syntax?

A. squares = [x * x for x in items] ✓

B. squares = [for x in items: x * x]

A list comprehension has the form [expression for variable in items]. The colon style belongs to a normal for loop, not a comprehension.

5. Given names = ['ava', 'liam'], which comprehension makes ['AVA', 'LIAM']?

A. [n.upper() for n in names] ✓

B. [n for n in names.upper()]

This transforms each item, so call .upper() on each n at the start. Calling .upper() on the list itself causes an AttributeError, because lists have no upper() method.

Match the Pairs

[x * 2 for x in nums] → Builds a new list with every number in nums doubled

[x for x in nums if x > 10] → Builds a new list with only the numbers in nums greater than 10

A list comprehension copies every item from a loop into a new list in one expression → copy = [x for x in items]

A list comprehension can transform each item, for example by making each name uppercase → upper = [n.upper() for n in names]

evens = [x for x in nums where x % 2 == 0] → 'where' is not Python. A condition in a list comprehension uses the keyword 'if'.

doubled = (x * 2 for x in nums) → Round brackets make a generator, not a list. Square brackets are needed to build a list.

Key Term

[x for x in items]

A list comprehension builds a new list from a loop in one expression. Add 'if condition' to filter items, or change x (e.g. x * 2) to transform each item.

Fill in the Blanks

1. doubled = [x * 2 for x in nums]

Hint: Type the keyword that starts the loop part inside the square brackets.

2. evens = [n for n in nums if n % 2 == 0]

Hint: Type the keyword that adds a condition to filter which items are kept.

3. squares = [n * n for n in range(1, 6)]

Hint: Type the expression at the front that transforms each item into its square.

4. marks = [55, 82, 47, 90] passes = [m for m in marks if m >= 50]

Hint: Type the comparison operator so only marks of 50 or more are kept.

5. names = ["ava", "liam"] big = [n.upper() for n in names]

Hint: Type the string method name that changes each item to capital letters.

Spot the Error

Question 1

A xs = [3, 8, 1, 9, 6]

B # keep numbers bigger than 5

C hi = [x for x xs if x > 5]

D print(hi)

Line C — The keyword 'in' is missing. A list comprehension needs 'for x in xs' to loop over the list, so the line should be hi = [x for x in xs if x > 5].

Question 2

A prices = [5, 10, 20]

B # double every price

C dbl = (p * 2 for p in prices)

D print(dbl)

Line C — Round brackets create a generator, not a list. A list comprehension uses square brackets: dbl = [p * 2 for p in prices].

Question 3

A pets = ['cat', 'dog', 'emu']

B up = [p.upper for p in pets]

C print(up)

Line B — upper is a method, so it needs brackets to be called. Without them the list holds method objects instead of uppercase text. It should be p.upper().

Question 4

A nums = [1, 2, 3, 4]

B sq = [n for n in nums]

C print(sq)

Line B — The expression at the front says how each item is transformed. Using just n copies the list unchanged. To square each item it should be n * n.

Question 5

A words = ['hi', 'yes', 'hello']

B print('Lengths:')

C # one length per word

D lens = [len(w) for w in word]

E print(lens)

Line D — The list is called words, not word, so Python raises a NameError. The loop part must name the real list: for w in words.

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