def search_for_vowels():

[DeaD_EyE]

Gives a Traceback name error when trying to invoke: "name 'search4vowels' not defined.

This happens if you call your function, before it's defined.

Example:

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foo() # <- NameError!     def foo():     pass

Error:
NameError: name 'foo' is not defined

This works:

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def foo():     pass     foo() # <- Name exists and points to the memory address, where the function lives

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By the way, your code works:

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In [2]: def search4vowels(word):    ...:     vowels = set('aeiou')    ...:     found = vowels.intersection(set(word))    ...:     for vowel in found:    ...:         print(vowel)    ...:   In [3]: search4vowels('Hello World') o e   In [4]: