How do I generate a variable sized list from a given list using Hypothesis?

Question

For property-based testing, given a fixed list of values, I need to generate a variable-sized list where order is important and duplicates are allowed. For example, if my fixed list is

texts = ['t1', 't2', 't3', 't4']

I would like to generate different variations, e.g.

['t2']
['t4', 't1'] # Subset and different order
[]
['t3', 't1', 't2'] # Different order
['t4', 't4', 't4', 't1'] # Repetition of t4
['t1', 't2', 't1'] # Repetition but at different location
['t1', 't2']
['t2', 't1'] # different order from the one above and considered different.

What I have managed to use currently is the permutations strategy

from hypothesis import given, strategies as st

@given(st.permutations(texts))
def test_x(some_text):
   ...
   pass

But that does not give me variable size, repetitions

Other requirements:

How can I specify a maximum variable list of 20?

lists(sampled_from(texts), max_size=20)? I'm just not sure what you mean by the "order is important" requirement. Could you clarify? — Georgy
– Georgy, Commented Jul 23, 2020 at 20:23
@Georgy Thanks! I meant that the strategy should serve ['t1', 't2'] as well as ["t2", "t1"] i.e permutations. — RAbraham
– RAbraham, Commented Jul 23, 2020 at 21:25

Georgy · Accepted Answer · 2020-07-24 10:18:22Z

4

You are looking for a combination of the lists and the sampled_from strategies:

from hypothesis import strategies as st

texts = ['t1', 't2', 't3', 't4']
lists_from_texts = st.lists(st.sampled_from(texts), max_size=20)

...

@given(lists_from_texts)
def test_x(some_text):
    ...

or if you want to be able to change the source list for different tests:

from typing import List


def lists_from_texts(source: List[str]) -> st.SearchStrategy[List[str]]:
    return st.lists(st.sampled_from(source), max_size=20)

...

@given(lists_from_texts(texts))
def test_x(some_text):
    ...

answered Jul 24, 2020 at 10:18

Georgy

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Comments

John Gordon · Accepted Answer · 2020-07-23 20:17:13Z

-1

Since you want at most 20 items, generate a random number from 1 to 20:

import random
size = random.randint(1,20)

Then use that number to make N number of independent choices from your source list:

texts = ['t1', 't2', 't3', 't4']
random_texts = []
for _ in range(size):
    random_texts.append(random.choice(texts))

edited Jul 23, 2020 at 20:17

answered Jul 23, 2020 at 19:02

John Gordon

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1 Comment

RAbraham Over a year ago

I wish I could select this answer too. Though I was using this question as a means to learn python-hypothesis, you showed me something I could use in other places as well. Thanks. upvoted.

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