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Validate string

I am trying to solve the following problem:

Given a string str, the task is to check if the string is a valid identifier or not. In order to qualify as a valid identifier, the string must satisfy the following conditions:

It must start with either underscore(_) or any of the characters from the ranges [‘a’, ‘z’] and [‘A’, ‘Z’]. There must not be any white space in the string. And, all the subsequent characters after the first character must not consist of any special characters like ,ドル #, % etc.

Examples:

Input: str= "_code123"

Output: Valid

Input: str = "123code_"

Output: Invalid

My attempted solution, which works:

def isValid(str1, n):
 # If first character is invalid
 if (((ord(str1[0]) >= ord('a') and
 ord(str1[0]) <= ord('z')) or
 (ord(str1[0]) >= ord('A') and
 ord(str1[0]) <= ord('Z')) or
 ord(str1[0]) == ord('_')) == False):
 return False
 # Traverse the string for the rest of the characters
 for i in range(1, len(str1)):
 if (((ord(str1[i]) >= ord('a') and
 ord(str1[i]) <= ord('z')) or
 (ord(str1[i]) >= ord('A') and
 ord(str1[i]) <= ord('Z')) or
 (ord(str1[i]) >= ord('0') and
 ord(str1[i]) <= ord('9')) or
 ord(str1[i]) == ord('_')) == False):
 return False
 # is a valid identifier
 return True
# Driver code
str1 = "_code123"
n = len(str1)
if (isValid(str1, n)):
 print("Valid")
else:
 print("Invalid")

How can I make the above code follow OOP principles, without so many nested ifs?

Answer*

There is nothing to say that nested ifs break OOP principles.

On the other hand, Python has certain features to help you out. First, you don't need to use `ord` so much because comparing two one-character strings does the same thing:

 ord('a') < ord('b')

is equivalent to

 'a' < 'b'

Second, Python allows you to chain math comparison operators, so instead of

 'a' < 'b' and 'b' < 'c''

you can just type

 'a' < 'b' < 'c'

But there is still an easier way: using the isalpha and isalphanumeric methods, you can avoid using the comparison operators, converting your code to:

Also, Python allows you to work directly on each character while looping by using the `for c in str` construct, saving you much indexing.

A more natural implementation of your code would be:

```py
def is_valid(str1, n):
 if not (str1[0] == '_' or str1[0].isalpha()):
 return False

 for c in str1[1:]:
 if not (c == '_' or c.isalnum()):
 return False

 return True
```

You could even use a list comprehension for the second part, by doing:


 if not all(c == '_' or c.isalnum() for c in str1[1:]):
 return False

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Validate string

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