Hypothesis: from_regex generates an odd amount of unexpected characters

[rlgomes]

Here is the example pytest test:

from hypothesis import given, strategies

@given(strategies.from_regex(r'\d{1}'))
def test_func(number):
    print(number)

When executed you'd expect to see 10 outputs from 0 to 9 right ? but what you end up seeing is a whole lot of unprintable characters and even letters in the output:

> pytest -s test.py
========================================================================== test session starts ==========================================================================
platform linux2 -- Python 2.7.13, pytest-3.2.1, py-1.4.34, pluggy-0.4.0 -- /home/rlgomes/workspace/python3/setup.py/env/bin/python2
cachedir: .cache
rootdir: /home/rlgomes/workspace/python3/setup.py, inifile: setup.cfg
plugins: cov-2.5.1, hypothesis-3.23.2
collected 1 item                                                                                                                                                         

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PASSED

======================================================================= 1 passed in 0.22 seconds ========================================================================

Currently running:

> python --version
Python 2.7.13
> pip freeze | grep hypothesis
hypothesis==3.23.2

Hopefully its something on my side but seems odd it would generate characters outside the scope of what that regex is expressing. I even changed the regex to be [0-9]{1} and I still get all of those other characters and the same thing happens if you do something like [a-c].

[Zac-HD]

from_regex() generates "strings that contain a match for the given regex" - so if you want to exclude a prefix or suffix, you'll need to use regex boundary markers.

@given(strategies.from_regex(r'\A\d{1}\Z'))
def test_func(number):
    print(number)

This - with the boundary markers - will generate hundreds of strings containing a single digit. Note that this is psudeo-random so some strings may be repeated... and that if you use the re.UNICODE flag or run this test under Python 3 there are many non-arabic numerals that can be generated!

[rlgomes]

Thanks @Zac-HD for the quick response! I should have remember the boundary markers :cry:

[Zac-HD]

No worries! Glad I could help 😄