python list comprehension

Python list comprehension is pretty useful at times to make code easier to read. There are typically 2 pattern for using list comprehension.

Pattern 1 (filtering)

[expression for item in iterable if condition]

Example:

nums = [1, 2, 3, 4, 5]
evens = [n for n in nums if n % 2 == 0]
print(evens)  # [2, 4]

A filtering

# filtering
[x for x in items if x > 0] filter → include only x > 0

In filtering we can chain multiple condition

nums = range(10)
filtered = [n for n in nums if n % 2 == 0 if n > 3]
print(filtered)  # [4, 6, 8]

Pattern 2 (transform)

[expression_if_true if condition else expression_if_false for item in iterable]

nums = [1, 2, 3, 4]
labels = ["even" if n % 2 == 0 else "odd" for n in nums]
print(labels)  # ['odd', 'even', 'odd', 'even']

A transformation construct

[x if x > 0 else 0 for x in items]  transform → replace negatives with 0

And here is an example to illustrate this

nums = [-2, -1, 0, 1, 2]
print([x for x in nums if x > 0])        # [1, 2]
print([x if x > 0 else 0 for x in nums]) # [0, 0, 0, 1, 2]

Pattern 3 - combining filter and transformation together

We can combine filter and tranformation with "if" filter at the end, for example

nums = [1, 2, 3, 4, 5, 6]
result = [n**2 if n % 2 == 0 else 0 for n in nums if n > 2]
print(result)  # [0, 16, 0, 36]

So what is happening are, we

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