Self in Python Syntax Error: Invalid Syntax, String's Attributes Inexisting?

Question

I have a little problem here; how do I make a self argument in Python? I want to be able to create an function that works like this:

'Hello, World!'.Write()

or at least something close to that. This is my code so far:

def Write(self):
  print(self)
'Hello, World!'.Write()

I got this as a syntax:

  File "main.py", line 3, in <module>
    'Hello, World!'.Write()
AttributeError: 'str' object has no attribute 'Write'
python3 exited with code 1...

so I tried changing the last line to Write.'Hello, World!'() instead. Now I have another error:

File "main.py", line 3
    Write.'Hello, world!'()
                        ^
SyntaxError: invalid syntax
python3 exited with code 1...

Something definitely isn't working. Did I miss one or a few steps? Is there something I am not seeing? Should I even use self?

"Something definitely isn't working. Did I miss one or a few steps?"—yes, you appear not to have read any documentation at all about how Python works. `self` doesn't do _anything_ like you seem to think it does. If you're trying to guess at how Python works based on experience with another language (Ruby, perhaps?) you're likely to have problems. Same if you're just making up syntax. Go through the tutorial as Justin suggests. At the very least, skim it. — ChrisGPT was on strike, Dec 30 '20 at 20:08
Have a read through [Can you monkey patch methods on core types in Python?](https://stackoverflow.com/q/192649/354577) — ChrisGPT was on strike, Dec 30 '20 at 20:34

score 1 · Accepted Answer · answered Dec 30 '20 at 20:50

Under normal circumstances, you can't add methods to an existing builtin object. Instead, the simple approach is to write a function that accepts the object you want to operate on:

def write(self):
    print(self)

You can call an argument whatever you want, including self. Now you can call it like this:

write('Hello World!')

If you were dealing with classes defined in Python rather than a builtin like str, you could try monkey-patching your method right in:

class MyType:
    def __init__(self, value):
        self.value = value

To monkey-patch the class, so all instances see the method:

MyType.write = write
MyType('Hello World!').write()

To monkey-patch just a single instance:

instance = MyType('Hello World!')
instance.write = write.__get__(instance)
instance.write()

All that being said, the proper approach to modifying existing classes with new functionality is to subclass. You are free to extend str with any functionality you see fit:

class MyType(str):
    def write(self):
        print(self)

The only thing that will differ from your example is that your class is not special and can no longer be initialized from a literal. In all other respects, it will behave like a string with a write method:

s = MyType('Hello World!')
s.write()

score 0 · Answer 2 · edited Jan 16 '21 at 20:59

0

I think this is a very complex solution and requires extended skills in Python. But it is possible.

class EnhancedString(object):
     def __init__(self, *args, **kwargs):
         self.s = str(*args, **kwargs)

     def Write(self):
         print(self.s);

mystring = EnhancedString('Hello, World!')
mystring.Write()

edited Jan 16 '21 at 20:59

Adrian Mole

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answered Dec 30 '20 at 20:14

wuerfelfreak

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1

Although not recommended – wuerfelfreak Dec 30 '20 at 20:14

Self in Python Syntax Error: Invalid Syntax, String's Attributes Inexisting?

2 Answers2