Membership test

Pythonic way of checking

def contains(haystack, needle):
    if needle not in haystack:
        raise ValueError('Needle not found')

contains([23, 42, 0xbadc0ffee], 'needle')

---------------------------------------------------------------------------

ValueError                                Traceback (most recent call last)

<ipython-input-4-c0a79d64f7a4> in <module>
      3         raise ValueError('Needle not found')
      4 
----> 5 contains([23, 42, 0xbadc0ffee], 'needle')


<ipython-input-4-c0a79d64f7a4> in contains(haystack, needle)
      1 def contains(haystack, needle):
      2     if needle not in haystack:
----> 3         raise ValueError('Needle not found')
      4 
      5 contains([23, 42, 0xbadc0ffee], 'needle')


ValueError: Needle not found

With a loop

def contains(haystack, needle):
    for item in haystack:
        if item == needle:
            return
    raise ValueError('Needle not found')
contains([23, 42, 0xbadc0ffee], 'needle')

---------------------------------------------------------------------------

ValueError                                Traceback (most recent call last)

<ipython-input-6-c605d648b237> in <module>
      4             return
      5     raise ValueError('Needle not found')
----> 6 contains([23, 42, 0xbadc0ffee], 'needle')


<ipython-input-6-c605d648b237> in contains(haystack, needle)
      3         if item == needle:
      4             return
----> 5     raise ValueError('Needle not found')
      6 contains([23, 42, 0xbadc0ffee], 'needle')


ValueError: Needle not found

Use for loop with else branch

Little bit confusing though, so avoid

def contains(haystack, needle):
    """
    Throw a ValueError if `needle` not
    in `haystack`.
    """
    for item in haystack:
        if item == needle:
            break
    else:
        # The `else` here is a
        # "completion clause" that runs
        # only if the loop ran to completion
        # without hitting a `break` statement.
        raise ValueError('Needle not found')
        
contains([23, 42, 0xbadc0ffee], 'needle')

---------------------------------------------------------------------------

ValueError                                Traceback (most recent call last)

<ipython-input-5-0869a17c6175> in <module>
     14         raise ValueError('Needle not found')
     15 
---> 16 contains([23, 42, 0xbadc0ffee], 'needle')


<ipython-input-5-0869a17c6175> in contains(haystack, needle)
     12         # only if the loop ran to completion
     13         # without hitting a `break` statement.
---> 14         raise ValueError('Needle not found')
     15 
     16 contains([23, 42, 0xbadc0ffee], 'needle')


ValueError: Needle not found