How to return a subset of a list that matches a condition [duplicate]

Question

Let's say I have a list of ints:

listOfNumbers = range(100) 

And I want to return a list of the elements that meet a certain condition, say:

def meetsCondition(element):     return bool(element != 0 and element % 7 == 0) 

What's a Pythonic way to return a sub-list of element in a list for which meetsCondition(element) is True?

A naive approach:

def subList(inputList):     outputList = []      for element in inputList:         if meetsCondition(element):             outputList.append(element)      return outputList  divisibleBySeven = subList(listOfNumbers) 

Is there a simple way to do this, perhaps with a list comprehension or set() functions, and without the temporary outputList?

Answer 1

Use list comprehension,

divisibleBySeven = [num for num in inputList if num != 0 and num % 7 == 0] 

or you can use the meetsCondition also,

divisibleBySeven = [num for num in inputList if meetsCondition(num)] 

you can actually write the same condition with Python's truthy semantics, like this

divisibleBySeven = [num for num in inputList if num and num % 7] 

---

alternatively, you can use filter function with your meetsCondition, like this

divisibleBySeven = filter(meetsCondition, inputList) 

%timeit

listOfNumbers = range(1000000)  %timeit [num for num in listOfNumbers if meetsCondition(num)] [out]: 243 ms ± 4.51 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)  %timeit list(filter(meetsCondition, listOfNumbers)) [out]: 211 ms ± 4.19 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)