Each character replace from string in python

The problem is that str.replace replaces one substring with a replacement substring. That's what the error means: it wants a substring, and you gave it a list of separate characters.

You could loop over the replacement pairs, calling replace for each one:

>>> s = '123'
>>> for search, replace in zip(search_array, replace_array):
...     s = s.replace(search, replace)
>>> s
'abc'

Or you could just use str.translate, which actually does do what you want, although it requires a bit of setup:

>>> search_array = ["1", "2", "3", "4", "5", "6", "7", "8", "9", "0"]
>>> replace_array = ["a", "b", "c", "d", "d", "e", "f", "g", "h", "i"]
>>> trans = str.maketrans(dict(zip(search_array, replace_array)))
>>> '123'.translate(trans)
'abc'

Or, alternatively, and probably more readable:

>>> search = "1234567890"
>>> replace = "abcddefghi"
>>> trans = str.maketrans(search, replace)
>>> '123'.translate(trans)
'abc'

By the way, if it isn't intentional that you specified d twice in a row, it might be clearer (and harder to make that typo!) to specify the letters like this:

>>> replace = string.ascii_lowercase[:10]

You can use a dictionary instead. There's a few steps here:

1. We want to map every number to a character, so we can create a dictionary with the input characters as keys and the output characters as values.
2. You can't iterate a numerical input, so we have to convert it to a string to access each character in turn. In your example the value is already a string ('123') but there's no reason you can't make the function accept both numerical and string inputs.
3. The replacement is done in a list comprehension, which naturally returns a list. So you need to call join() to join all of the characters back together into a single string.

One approach is as follows:

def number2char(number):
    search_array = ["1", "2", "3", "4", "5", "6", "7", "8", "9", "0"]
    replace_array = ["a", "b", "c", "d", "d", "e", "f", "g", "h", "i"]
    replace_dict = dict(zip(search_array, replace_array))
    num_2_char = ''.join([replace_dict.get(char, '_') 
                          for char in str(number)])
    return num_2_char

a = number2char(123)

In this case I've used replace_dict.get(str(item), '_') so that the code doesn't crash if someone gives an input that isn't in the replace_dict; instead they just get back underscores. There's probably better ways to handle that, but it's safer than just trying to access the dictionary as replace_dict[item].

def number2char(number):
    search_array = ["1", "2", "3", "4", "5", "6", "7", "8", "9", "0"]
    replace_array = ["a", "b", "c", "d", "d", "e", "f", "g", "h", "i"]
    num_2_char=''
    for i in number:
        num_2_char +=  replace_array[search_array.index(i)]
    return num_2_char;