#lambda functions

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JavaScript's Lambda and Arrow Functions

A quick introduction to lambda functions in Python.

Below you find some examples of lambda functions in Python.

Documentation is added to the code directly – in the form of comments.

Lambda fucntions in Python are stated in accordance with the following syntax: lamda arguments : expression

# defining functions "cubing" with lambda operator

cubing = lambda x: x**3

#…

ESP32 / ESP8266 MicroPython: Applying filter function to lists

The objective of this post is to explain how to use the filter function with lists in MicroPython. This tutorial was tested both on the ESP32 and on the ESP8266. The tests on the ESP32 were performed using a DFRobot’s ESP-WROOM-32 device integrated in a ESP32 FireBeetle board.

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ESP32 / ESP8266 MicroPython: Lambda functions

The objective of this post is to explain how to use lambda functions in MicroPython and their difference regarding regular functions. This tutorial was tested both on the ESP32 and on the ESP8266. The tests on the ESP32 were performed using a DFRobot’s ESP-WROOM-32 device integrated in a ESP32 FireBeetle board.

(more…)

The issue

Recently, a colleague posted the following code snippet and asked for an explanation of why it didn't work as they expected:

x = [] for i in range(1, 10): x.append(lambda z: "%d" % i + z) print(list(x[i]("banzaii") for i in range(0, 9)))

which results in:

['9banzaii', '9banzaii', '9banzaii', '9banzaii', '9banzaii', '9banzaii', '9banzaii', '9banzaii', '9banzaii']

instead of the expected:

['1banzaii', '2banzaii', '3banzaii', '4banzaii', '5banzaii', '6banzaii', '7banzaii', '8banzaii', '9banzaii']

The explanation

The key is that the variable i is outside of the scope of the created lambda. Only the variable z is in the lambda scope. Essentially it would be like writing:

x=[] def fn(z): return "%d" % i + z for i in range(1, 10): x.append(fn)

which makes it a lot more obvious what is going on. The global variable i is used each time the function is actually called--which is after the for loop is executed and i is now 9.

The fix

The fix is simple: make sure that i is in the scope of the function that is appended to array x.

Solution 1: using a closure

def makefunc(i): def inner(z): return "%d" % i + z return inner x=[] for i in range(1, 10): x.append(makefunc(i))

In this example, a closure around the function inner passes i into the returned function's scope.

Solution 2: using lambda to create an anonymous closure

x=[] for i in range(1, 10): x.append((lambda n: lambda z: "%d" % n + z)(i))

Here, we are appending the result of calling a lambda function with the argument i which returns a lambda that uses that value. This is exactly equivalent to the previous example, but for people who like one-liners. :)

Solution 3: using a lambda with a default argument

x=[] for i in range(1, 10): x.append(lambda z, n=i: "%d" % n + z)