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About getattr()
#1
In[1]: getattr(None, '') is None
Traceback (most recent call last):
  File "C:\Users\myluo\AppData\Local\Programs\Python\Python36\lib\site-packages\IPython\core\interactiveshell.py", line 3265, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-8-dd6743d652c1>", line 1, in <module>
    getattr(None, '') is None
AttributeError: 'NoneType' object has no attribute ''

In[2]: getattr(None, '', None) is None
Out[2]: True
What's happend?
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#2
The first use of getattr returned an attribute error because an empty string was given and no default was given.
The second use of getattr was also given an empty string but because a default was provided the default value was returned instead.
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#3
(Mar-10-2019, 08:30 AM)Yoriz Wrote: The first use of getattr returned an attribute error because an empty string was given and no default was given. The second use of getattr was also given an empty string but because a default was provided the default value was returned instead.

def getattr(object, name, default=None)
The default argument has None as default value.
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#4
(Mar-11-2019, 02:35 AM)MingyuanLuo Wrote: The default argument has None as default value.

Where are you getting that from? It's not in the documentation. The documentation describes the behavior of getattr as Yoriz did, and as you saw.
Craig "Ichabod" O'Brien - xenomind.com
I wish you happiness.
Recommended Tutorials: BBCode, functions, classes, text adventures
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#5
Yoriz already gave you an answer. I add advice which may speed up learning process.

If you encounter something you don't understand (or something what is unexpected) I suggest always start with built-in help and documentation.

>>> help(getattr)
Help on built-in function getattr in module builtins:

getattr(...)
    getattr(object, name[, default]) -> value
    
    Get a named attribute from an object; getattr(x, 'y') is equivalent to x.y.
    When a default argument is given, it is returned when the attribute doesn't
    exist; without it, an exception is raised in that case.
(END)
Documentation at python.org:

getattr(object, name[, default])
Return the value of the named attribute of object. name must be a string. If the string is the name of one of the object’s attributes, the result is the value of that attribute. For example, getattr(x, 'foobar') is equivalent to x.foobar. If the named attribute does not exist, default is returned if provided, otherwise AttributeError is raised.

In ipython there is even less typing:

In [1]: getattr?                                                                                                                  
Docstring:
getattr(object, name[, default]) -> value

Get a named attribute from an object; getattr(x, 'y') is equivalent to x.y.
When a default argument is given, it is returned when the attribute doesn't
exist; without it, an exception is raised in that case.
Type:      builtin_function_or_method
I'm not 'in'-sane. Indeed, I am so far 'out' of sane that you appear a tiny blip on the distant coast of sanity. Bucky Katt, Get Fuzzy

Da Bishop: There's a dead bishop on the landing. I don't know who keeps bringing them in here. ....but society is to blame.
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#6
(Mar-11-2019, 02:50 AM)ichabod801 Wrote:
(Mar-11-2019, 02:35 AM)MingyuanLuo Wrote: The default argument has None as default value.

Where are you getting that from? It's not in the documentation. The documentation describes the behavior of getattr as Yoriz did, and as you saw.

I saw and understand the documentation. But I saw this in builtins.py
def getattr(object, name, default=None): # known special case of getattr
    """
    getattr(object, name[, default]) -> value
    
    Get a named attribute from an object; getattr(x, 'y') is equivalent to x.y.
    When a default argument is given, it is returned when the attribute doesn't
    exist; without it, an exception is raised in that case.
    """
    pass
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#7
(Mar-11-2019, 08:41 AM)perfringo Wrote: Yoriz already gave you an answer. I add advice which may speed up learning process.

If you encounter something you don't understand (or something what is unexpected) I suggest always start with built-in help and documentation.

>>> help(getattr)
Help on built-in function getattr in module builtins:

getattr(...)
    getattr(object, name[, default]) -> value
    
    Get a named attribute from an object; getattr(x, 'y') is equivalent to x.y.
    When a default argument is given, it is returned when the attribute doesn't
    exist; without it, an exception is raised in that case.
(END)
Documentation at python.org:

getattr(object, name[, default])
Return the value of the named attribute of object. name must be a string. If the string is the name of one of the object’s attributes, the result is the value of that attribute. For example, getattr(x, 'foobar') is equivalent to x.foobar. If the named attribute does not exist, default is returned if provided, otherwise AttributeError is raised.

In ipython there is even less typing:

In [1]: getattr?                                                                                                                  
Docstring:
getattr(object, name[, default]) -> value

Get a named attribute from an object; getattr(x, 'y') is equivalent to x.y.
When a default argument is given, it is returned when the attribute doesn't
exist; without it, an exception is raised in that case.
Type:      builtin_function_or_method

I saw and understand the documentation. But I saw this in builtins.py
def getattr(object, name, default=None): # known special case of getattr
    """
    getattr(object, name[, default]) -> value
    
    Get a named attribute from an object; getattr(x, 'y') is equivalent to x.y.
    When a default argument is given, it is returned when the attribute doesn't
    exist; without it, an exception is raised in that case.
    """
    pass
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#8
(Mar-20-2019, 10:35 AM)MingyuanLuo Wrote: I saw and understand the documentation. But I saw this in builtins.py

Then builtins.py is wrong, and the documentation is right, because the documentation describes how it actually works when you run it in Python.
Craig "Ichabod" O'Brien - xenomind.com
I wish you happiness.
Recommended Tutorials: BBCode, functions, classes, text adventures
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