如何在python中将变量更改为字符串？

Question

So I am trying to change a randomized variable to a string with a function, any ideas why this isn't working?

def letter(x):
    if x == 1:
            x = "A"
    elif x == 2:
            x = "C"
    elif x == 3:
            x = "G"
    elif x == 4:
            x = "T"
    else:
            print "Error" 
randint18= random.randrange(1,5)
letter(randint18)
print randint18 `

Answer 1

You have to return the value from the function, and assign it to a variable.

def letter(x):
    ...
    return x


randint18 = random.randrange(1, 5)
result = letter(randint18)
print result

Answer 2

You cannot alter the variable in place you must return it and capture the returned value.

import random

def letter(x):
    if x == 1:
            x = "A"
    elif x == 2:
            x = "C"
    elif x == 3:
            x = "G"
    elif x == 4:
            x = "T"
    else:
        print "Error" 
    return x  # return it here

randint18= random.randrange(1,5)
randint18 = letter(randint18)  # capture the returned value here 
print randint18

There is a simpler way to achieve what you want, using a dictionary to map the values.

import random

def letter(x):
    mapd = {1:'A', 2:'C', 3:'G', 4:'T'}
    return mapd.get(x, None)

randint18= random.randrange(1,5)
randint18 = letter(randint18)
print randint18

Answer 3

mine isn't a proper answer, which have been provided already, but a suggestion for improving your code. I'd do it in a comment, but the code formatting ain't good enough.

Why not use a dictionary for the mapping, instead of a sequence of if's? You could still place it in a function if you like:

letter = {1:'A', 2:'C', 3:'G', 4:'T'}
randint18 = random.randrange(1,5)
mapping = letter.get(randint18, 'Error')
print mapping

mind you, a list would be even more efficient, if the mapping started form zero:

letter = ['A', 'C', 'G', 'T']
randint18 = random.randrange(0,4)
try: # in case your random index were allowed to go past 3
 mapping = letter[randint18]
except IndexError:
 mapping = 'Error'
print mapping

Answer 4

You forgot to include a return in your function

def letter(x):
    if x == 1:
            x = "A"
    elif x == 2:
            x = "C"
    elif x == 3:
            x = "G"
    elif x == 4:
            x = "T"
    else:
            print "Error" 
    return x
randint18 = random.randrange(1,5)
returned_result = letter(randint18)
print returned_result

Answer 5

Add a return value of the function

return x

value_you_want = letter(randint18) ##add the return statement. Output will be saved to value_you_want

Please note that the variables defined inside a function are local to the function and cannot be accessed outside the scope of the function. You were expecting the value of x outside the function which is not possible. Just to check run your function and try to access the value in variable x. It will give error.

Traceback (most recent call last):
  File "<pyshell#0>", line 1, in <module>
    print x
NameError: name 'x' is not defined