Python – TypeError: ‘str’ object does not support item assignment (Python)

for-looppython

So this is what I'm trying to do:

input = ABCDEFG

***DEFG
A***EFG
AB***FG
ABC***G
ABCD***

and this is the code I wrote

def loop(input):
    output = input
    for index in range(0, len(input)-3):              #column length
        output[index:index +2] = '***'
        output[:index] = input[:index]
        output[index+4:] = input[index+4:]
        print output + '\n'

But I get the error: TypeError: 'str' object does not support item assignment

Help?

Best Answer

You cannot modify the contents of a string, you can only create a new string with the changes. So instead of the function above you'd want something like this

def loop(input):
    for index in range(0, len(input)-3):              #column length
        output = input[:index] + '***' + input[index+4:]
        print output

Related Solutions

Python – Does Python have a ternary conditional operator

Yes, it was added in version 2.5. The expression syntax is:

a if condition else b

First condition is evaluated, then exactly one of either a or b is evaluated and returned based on the Boolean value of condition. If condition evaluates to True, then a is evaluated and returned but b is ignored, or else when b is evaluated and returned but a is ignored.

This allows short-circuiting because when condition is true only a is evaluated and b is not evaluated at all, but when condition is false only b is evaluated and a is not evaluated at all.

For example:

>>> 'true' if True else 'false'
'true'
>>> 'true' if False else 'false'
'false'

Note that conditionals are an expression, not a statement. This means you can't use assignment statements or pass or other statements within a conditional expression:

>>> pass if False else x = 3
  File "<stdin>", line 1
    pass if False else x = 3
          ^
SyntaxError: invalid syntax

You can, however, use conditional expressions to assign a variable like so:

x = a if True else b

Think of the conditional expression as switching between two values. It is very useful when you're in a 'one value or another' situation, it but doesn't do much else.

If you need to use statements, you have to use a normal if statement instead of a conditional expression.

Keep in mind that it's frowned upon by some Pythonistas for several reasons:

The order of the arguments is different from those of the classic condition ? a : b ternary operator from many other languages (such as C, C++, Go, Perl, Ruby, Java, Javascript, etc.), which may lead to bugs when people unfamiliar with Python's "surprising" behaviour use it (they may reverse the argument order).
Some find it "unwieldy", since it goes contrary to the normal flow of thought (thinking of the condition first and then the effects).
Stylistic reasons. (Although the 'inline if' can be really useful, and make your script more concise, it really does complicate your code)

If you're having trouble remembering the order, then remember that when read aloud, you (almost) say what you mean. For example, x = 4 if b > 8 else 9 is read aloud as x will be 4 if b is greater than 8 otherwise 9.

Official documentation:

Python – How to know if an object has an attribute in Python

Try hasattr():

if hasattr(a, 'property'):
    a.property

See zweiterlinde's answer below, who offers good advice about asking forgiveness! A very pythonic approach!

The general practice in python is that, if the property is likely to be there most of the time, simply call it and either let the exception propagate, or trap it with a try/except block. This will likely be faster than hasattr. If the property is likely to not be there most of the time, or you're not sure, using hasattr will probably be faster than repeatedly falling into an exception block.

Best Answer

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Python – How to know if an object has an attribute in Python

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