Lambdas and closures

The function as a value: anonymous function, function passed as an argument, function returning a function, closure, captured variables and the late-binding trap in a loop.

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Anonymous function (lambda): what is it?
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A function defined by a single expression, without giving it a name, right where it is needed: lambda x: x * 2.
In Python, a lambda's body is a single expression whose value is returned: no statement, no return. As soon as the logic grows, a named function is more readable.
What does this program print?

```python
fs = [lambda: i for i in range(3)]
print([f() for f in fs])
`
- [2, 2, 2]
- [0, 1, 2]
- [3, 3, 3]
- [0, 0, 0]
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[2, 2, 2]
The three lambdas share the same variable i, read at call time: the loop left it at 2. To freeze each value: lambda i=i: i prints [0, 1, 2].
To sort words from longest to shortest, write sorted(words, key=lambda m: …).
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To sort words from longest to shortest, write sorted(words, key=lambda m: -len(m)).
key receives each element and returns the value to compare on; the minus sign reverses the order. You can also write reverse=True with key=len.
Higher-order function: what is it?
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A function that takes another function as an argument, or returns one.
sorted(list, key=...), map and filter are examples: the function passed describes what varies, the receiving function handles the loop.
Three buttons are created in a for n in range(3) loop, each with lambda: show(n). All display 2. Which fix is right?
- Replace range(3) by range(1, 4)
- Bind the value at creation: lambda n=n: show(n)
- Call show(n) at creation instead of putting it in the lambda
- Replace the lambda by a list comprehension
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Bind the value at creation: lambda n=n: show(n)
The default value n=n is evaluated when each lambda is created and freezes the current value. Calling show(n) right away would run it instead of waiting for the click.

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