Using in in Python

pro_lang = ['C', 'C++', 'java', 'Perl']

lang_to_check = 'Python C Haskel JavaScript Perl'

for lang in lang_to_check.split():
    # check for value in list
    if lang in pro_lang:
        continue
    else:
        pro_lang.append(lang)

print pro_lang  #prints ['C', 'C++', 'java', 'Perl', 'Python', 'Haskel', 'JavaScript']