Python: Understanding _ STR __

Here is my understanding. If there is something wrong with me. Please be sure to tell me. Thank you!

In python, __str _ is generally in this format.

Class:

Def _ STR _ (Self ):

Return "this is in Str"

In fact, __str _ is called by the print function. Generally, it is a return object. This is represented in a string. If the STR () function is not used for conversion. When printing a class, print first calls the _ STR __defined in the class, for example, str. py.

#!/usr/bin/env python                                                                                                                                                                                 class strtest:    def __init__(self):        print "init: this is only test"    def __str__(self):        return "str: this is only test"if __name__ == "__main__":    st=strtest()    print st

$./Str. py

Init: This is only test

STR: This is only test

As shown in the preceding example, when a strtest instance st is printed, the __str _ function is called.

In fact, basically all objects in Python have _ STR _ by default for the print function. For example, the _ STR __in the dictionary can be found in the red part:

>>> Dir ({})
['_ Class _', '_ CMP _', '_ ins INS _', '_ delattr __', '_ delitem _', '_ Doc _', '_ eq _', '_ format _', '_ Ge __', '_ getattribute _', '_ getitem _', '_ GT _', '_ Hash _', '_ init __', '_ ITER _', '_ Le _', '_ Len _', '_ LT _', '_ ne __', '_ new _', '_ reduce _', '_ performance_ex _', '_ repr _', '_ setattr __', '_ setitem _', '_ sizeof _', '_ STR _', '_ subclasshook _', 'clear', 'copy ', 'fromkeys ', 'get', 'has _ key', 'items', 'iteritems ', 'iterkeys', 'itervalues', 'keys ', 'pop ', 'popitem', 'setdefault', 'update', 'values']

>>> T = {}

>>> T ['1'] = "hello"

>>> T ['2'] = "world"
>>> T # equals to print T
{'1': 'hello', '2': 'World '}
>>> T. _ STR __()
"{'1': 'hello', '2': 'World '}"

We can see that print T is the same as T. _ STR. Only _ STR _ () outputs the dictionary content as a string.

Let's take a look at the example below. If the returned result in function _ STR _ is not a string, see str1.py

#!/us/bin/env/python                                                                                                                                                                                        #__metaclass__ = type#if __name__ == "__main__":class strtest:    def __init__(self):        self.val = 1    def __str__(self):        return self.valif __name__ == "__main__":    st=strtest()    print st

$./Str1.py

Traceback (most recent call last ):
File "./Str. py", line 12, in <module>
Print St
Typeerror: _ STR _ returned non-string (type INT)

Error message: __str _ A non-string is returned. At this time, we should do this: see str2.py

#!/usr/bin/env python                                                                                                                                                                                        #__metaclass__ = type#if __name__ == "__main__":class strtest:    def __init__(self):        self.val = 1    def __str__(self):        return str(self.val)if __name__ == "__main__":    st=strtest()    print st

$./Str2.py

1

We use STR () to convert the integer type to the struct type.