In Python, there is no existence of “Private” instance variables that cannot be accessed except inside an object. However, a convention is being followed by most Python code and coders i.e., a name prefixed with an underscore, For e.g. _prutor should be treated as a non-public part of the API or any Python code, whether it is a function, a method, or a data member. While going through this we would also try to understand the concept of various forms of trailing underscores, for e.g., for _ in range(10), __init__(self).

Mangling and how it works

In Python, there is something called name mangling, which means that there is a limited support for a valid use-case for class-private members basically to avoid name clashes of names with names defined by subclasses. Any identifier of the form __prutor (at least two leading underscores or at most one trailing underscore) is replaced with _classname__prutor, where classname is the current class name with a leading underscore(s) stripped. As long as it occurs within the definition of the class, this mangling is done. This is helpful for letting subclasses override methods without breaking intraclass method calls.
Let’s look at this example and try to find out how this underscore works:

# Python code to illustrate how mangling works
class Map:
def __init__(self, iterate):
self.list = []
self.__prutor(iterate)
def prutor(self, iterate):
for item in iterate:
self.list.append(item)

# private copy of original prutor() method
__prutor = prutor

class MapSubclass(Map):

# provides new signature for prutor() but
# does not break __init__()
def prutor(self, key, value):
for i in zip(keys, value):
self.list.append(i)
The mangling rules are designed mostly to avoid accidents but it is still possible to access or modify a variable that is considered private. This can even be useful in special circumstances, such as in the debugger.

_Single Leading Underscores

So basically one underline in the beginning of a method, function, or data member means you shouldn’t access this method because it’s not part of the API. Let’s look at this snippet of code:

# Python code to illustrate
# how single underscore works
def _get_errors(self):
if self._errors is None:
self.full_clean()
return self._errors

errors = property(_get_errors)
The snippet is taken from the Django source code (django/forms/forms.py). This suggests that errors is a property, and it’s also a part of the API, but the method, _get_errors, is “private”, so one shouldn’t access it.

__Double Leading Underscores

Two underlines, in the beginning, cause a lot of confusion. This is about syntax rather than a convention. double underscore will mangle the attribute names of a class to avoid conflicts of attribute names between classes. For example:

# Python code to illustrate how double
# underscore at the beginning works
class prutor:
def _single_method(self):
pass
def __double_method(self): # for mangling
pass
class Pyth(prutor):
def __double_method(self): # for mangling
pass
__Double leading and Double trailing underscores__

There’s another case of double leading and trailing underscores. We follow this while using special variables or methods (called “magic method”) such as__len__, __init__. These methods provide special syntactic features to the names. For example, __file__ indicates the location of the Python file, __eq__ is executed when a == b expression is executed.
Example:

# Python code to illustrate double leading and
# double trailing underscore works
class prutor:

# '__init__' for initializing, this is a
# special method
def __init__(self, ab):
self.ab = ab

# custom special method. try not to use it
def __custom__(self):
pass