check if a string has only white characters in python

Question

What is the easyest way to check if a String has only white characters, like \r \t \n " "?

Answer 1

The Desired Behavior

The following contains a description of the desired behavior....

---

Specific Behavior

INPUT	DESCRIPTION	OUTPUT
"a b c"	Letters and Spaces mixed together.	False
"abc"	Letters Only	False
" "	Three Spaces	True
""	Empty String	No Exception is raised. It undefined whether the return value is True or False
" "	One Space	True
"\t"	Tab-character	True
"\r"	Carriage Return	True
"\n"	Line-Feed	True
"\v"	Vertical Tab	True
"\f"	form-feed	True
"\t\v\t\rn"	Mixture of tabs and other white characters	True

---

General Behavior

In ASCII, the whitespace characters are:

• space ( ' ' )
• tab ( '\t' )
• carriage return ( '\r' )
• newline ( '\n' )
• vertical tab ( '\v' )
• formfeed ( '\f' )

def is_pure_white(itext:str) -> bool:
    """
        DOCUMENTATION

        Returns true if all characters in the string are 
        spaces, or line-feeds, or tabs, or other things that look
        white when the text is printed onto a white sheet of paper.  
        
        The input parameter `itext` is any string of text. 
 
        The output is a Boolean (True or False)        
    """
    raise NotImplementedError() 

---

Behavior for the Empty String

Note that the empty string "" is a strange corner case.

Consider...

CASE NUMBER	MATHEMATICIANS SYMBOL	DESCIPTION	BEHAVIOR FOR EMPTY STRING
1 of 2	∃	There exists at least one character in the string which is not a white-space character	False for the empty string
2 of 2	∀	Either the string is empty, or there is at least one character in the string, and that character is white-space, and all other characters in the string are also white-space.	True for the empty string

---

Implementation One
Using a For Loop

Implementation 1.1
One Return Statement Only

def is_pure_white(itext:str)  -> bool:
    r = True
    for ch in itext:
        if ch not in [' ', '\t', '\r', '\n', '\v', '\f']: 
            r = False
            break 
    return r    

---

Implementation 1.2
Two Return Statements

def is_pure_white(itext:str)  -> bool:
    for ch in itext:
        if ch not in [' ', '\t', '\r', '\n', '\v', '\f']: 
            return False
    return True   

---

Implementation Two
Remove Leading and Trailing White-Space

def is_pure_white(itext:str)  -> bool:
    """
        [
            INSTRUCTIONS 
            ON *HOW TO USE* THIS FUNCTION GO HERE

            NOTES ABOUT *HOW IT WORKS* ARE FORBIDDEN HERE
            DO NOT WRITE ABOUT THE INNER WORKINGS OR IMPLEMENTATION
        ]
    """
    # BEGIN DOCUMENTATION FOR IMPLEMENTATION (HOW IT WORKS)  
    #
    #     FIRST....
    #         Remove Leading and Trailing White-Space
    #
    #     SECONDLY....
    #         Check IF the Length of the string is zero.   
    # 
    # END DOCUMENTATION FOR IMPLEMENTATION (HOW IT WORKS)  
 
    text = itext.strip()
        if len(text) > 1:
            return False
        # else:
        #     empty string or all white space characters 
    return True   

---

Option Three
Using the Regular Expression Engine

import re # regular expression library 

def is_pure_white(itext:str) -> bool:
    pattern = "\s"
    spaces = re.findall(pattern, itext)
    status = len(spaces) == len(itext)
    return status

---

Option Four
Predicate Dispatching

This one requires a special library that most people don't have.

import casedispatching as kispatch 

@kispatch(lambda stryng: min(2, len(stryng)) ) 
def is_pure_white(itext:str) -> bool:
    praise NotImplementedError()  

###############################################################

@is_pure_white.register(0) 
def _(itext:str) -> bool:
    """ empty string """ 
    return True 

###############################################################

@is_pure_white.register(1) 
def _(ch:str) -> bool:
    status = ch in [' ', '\t', '\r', '\n', '\v', '\f']   
    return status   

################################################################

@is_pure_white.register(2) 
def _(itext:str) -> bool:
    # recursive implementation 
    for ch in itext:
        if not is_pure_white(ch) 
            return False
    return True  

---

A Simple Driver used for Testing Purposes

You can test your code, my code, or someone else's code using the following driver:

txt2 = "\t\t \n"
txt1 = "The rain in Spain\n\r"

cases = [txt1, txt2]

for case in cases:
    result = is_pure_white(case)
    print(repr(case).ljust(40), result)

Answer 2

The isspace() method on strings tells you this:

>>> "   ".isspace()
True
>>> " x  ".isspace()
False

Another option is to strip the ends:

if not s.strip():
    print "It was all whitespace!"