spisok=[{'a':'1','b':'2'},{'a':'3','b':'4'}]

for i in range(len(spisok)):
    if spisok[i]['a'] == stroka_poiska:
        print('True')

def chk_for_val(lst, key, val):
    for d in lst:
        if d.get(key) == val:
            return True
    return False

In [138]: chk_for_val(lst, 'a', '3')
Out[138]: True

In [139]: chk_for_val(lst, 'X', '3')
Out[139]: False

In [17]: array = [{'a':'1','b':'2'},{'a':'3','b':'4'}]

In [18]: def search(dictionary, key, value):
    ...:     verbose_data = sum(map(list, map(dict.items, dictionary)), [])
    ...:     return (key, value) in verbose_data
    ...:

In [19]: search(array, 'a', '1')
Out[19]: True

In [20]: search(array, 'a', '4')
Out[20]: False

In [27]: search = lambda array, key, value: any(map(lambda x: (key, value) in x.items(), array))

In [28]: search(array, 'a', '1')
Out[28]: True

In [29]: search(array, 'A', '1')
Out[29]: False

for i in range(len(spisok)):
    for key in spisok[i]:
        if spisok[i][key]=='4':
            print(spisok[i][key])