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

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