{"id":1791,"date":"2019-04-05T07:33:39","date_gmt":"2019-04-05T06:33:39","guid":{"rendered":"https:\/\/rosetta.vn\/short\/2019\/04\/05\/understanding-the-underscore-_-of-python-hacker-noon\/"},"modified":"2019-04-05T07:33:39","modified_gmt":"2019-04-05T06:33:39","slug":"understanding-the-underscore-_-of-python-hacker-noon","status":"publish","type":"post","link":"https:\/\/rosetta.vn\/short\/2019\/04\/05\/understanding-the-underscore-_-of-python-hacker-noon\/","title":{"rendered":"Understanding the underscore( _ ) of Python \u2013 Hacker Noon"},"content":{"rendered":"
\n

There are 5 cases for using the <\/span>underscore<\/em>in Python.<\/p>\n

    \n
  1. For storing the value of last expression in interpreter.<\/li>\n
  2. For ignoring the specific values. (so-called \u201cI don\u2019t care\u201d)<\/li>\n
  3. To give special meanings and functions to name of vartiables or functions.<\/li>\n
  4. To use as \u2018Internationalization(i18n)\u2019 or \u2018Localization(l10n)\u2019 functions.<\/li>\n
  5. To separate the digits of number literal value.<\/li>\n<\/ol>\n

    Let\u2019s look at each case.<\/p>\n

    When used in interpreter<\/span><\/h3>\n

    The python interpreter stores the last expression value to the special variable called \u2018_\u2019. This feature has been used in standard CPython interpreter first and you could use it in other Python interpreters too.<\/p>\n

    >>> 10 \n10 \n>>> _ \n10 \n>>> _ * 3 \n30 \n>>> _ * 20 \n600<\/pre>\n
    For Ignoring the values<\/span><\/h3>\n
    The <\/span>underscore<\/em> <\/span>is also used for ignoring the specific values. If you don\u2019t need the specific values or the values are not used, just assign the values to <\/span>underscore<\/em>.<\/p>\n
    # Ignore a value when unpacking\n<\/em>x, _, y = (1, 2, 3) # x = 1, y = 3<\/em><\/pre>\n
    # Ignore the multiple values. It is called \"Extended Unpacking\" which is available in only Python 3.x<\/em>\nx, *_, y = (1, 2, 3, 4, 5) # x = 1, y = 5<\/em><\/pre>\n
    # Ignore the index\n<\/em>for _ in range(10):     \n    do_something()<\/pre>\n
    # Ignore a value of specific location<\/em>\nfor _, val in list_of_tuple:\n    do_something()<\/pre>\n
    Give special meanings to name of variables and functions<\/span><\/h3>\n
    The <\/span>underscore<\/em> <\/span>may be most used in \u2018naming\u2019. The PEP8 which is Python convention guideline introduces the following 4 naming cases.<\/p>\n
    _single_leading_underscore
    \n<\/span>This convention is used for declaringprivate<\/em><\/span> <\/span>variables, functions, methods and classes in a module. Anything with this convention are ignored in <\/span>from module import *<\/code>. 
    \nHowever, of course, Python does not supports <\/span>truly private, <\/span><\/em><\/span>so we can not force somethings private ones and also can call it directly from other modules. So sometimes we say it \u201cweak internal use indicator\u201d.<\/p>\n
    _internal_name = 'one_nodule' # private variable\n_internal_version = '1.0' # private variable<\/pre>\n
    \nclass _Base: # private class\n    _hidden_factor = 2 # private variable<\/pre>\n
        def __init__(self, price):\n        self._price = price<\/pre>\n
        def _double_price(self): # private method\n        return self._price * self._hidden_factor<\/pre>\n
        def get_double_price(self):\n        return self._double_price()<\/pre>\n
    single_trailing_underscore_
    \n<\/span>This convention could be used for avoiding conflict with Python keywords or built-ins. You might not use it often.<\/p>\n
    Tkinter.Toplevel(master, class_='ClassName') # Avoid conflict with 'class' keyword<\/pre>\n
    list_ = List.objects.get(1) # Avoid conflict with 'list' built-in type<\/pre>\n
    __double_leading_underscore
    \n<\/span>This is about syntax rather than a convention. <\/span>double underscore <\/span><\/em>will<\/em>mangle the attribute names of a class to avoid conflicts of attribute names between classes. (so-called \u201cmangling\u201d that means that the compiler or interpreter modify the variables or function names with some rules, not use as it is) 
    \nThe mangling rule of Python is adding the \u201c_ClassName\u201d to front of attribute names are declared with <\/span>double underscore. 
    \n<\/em>That is, if you write method named \u201c__method\u201d in a class, the name will be mangled in \u201c_ClassName__method\u201d form.<\/p>\n
    class A:\n    def _single_method(self):\n        pass<\/pre>\n
        def __double_method(self): # for mangling\n        pass<\/pre>\n
    class B(A):\n    def __double_method(self): # for mangling\n        pass<\/pre>\n
    Because of the attributes named withdouble underscore <\/span><\/em>will be mangled like above, we can not access it with \u201cClassName.__method\u201d. Sometimes, some people use it as like real private ones using these features, but it is not for private and not recommended for that. For more details, read <\/span>\t<\/svg>"); background-color: transparent; background-size: 1px 1px; background-position: 0px calc(1em + 1px); background-repeat: repeat-x;”>Python Naming<\/a>.<\/p>\n
    __double_leading_and_trailing_underscore__
    \n<\/span>This convention is used for special variables or methods (so-called \u201cmagic method\u201d) such as__init__<\/code>, <\/span>__len__<\/code>. These methods provides special syntactic features or does special things. For example, <\/span>__file__<\/code> <\/span>indicates the location of Python file, <\/span>__eq__<\/code> <\/span>is executed when <\/span>a == b<\/code> <\/span>expression is excuted. 
    \nA user of course can make custom special method, it is very rare case, but often might modify the some built-in special methods. (e.g. You should initialize the class with <\/span>__init__<\/code> <\/span>that will be executed at first when a instance of class is created.)<\/p>\n
    class A:\n    def __init__(self, a): # use special method '__init__' for initializing\n        self.a = a<\/pre>\n
        def __custom__(self): # custom special method. you might almost do not use it\n        pass<\/pre>\n
    As Internationalization(i18n)\/Localization(l10n) functions<\/h3>\n
    It is just convention, does not have any syntactic functions. That is, the underscore does not means <\/span>i18n\/l10n<\/span>, and it is just a convention that binds thei18n\/l10n<\/span> <\/span>to <\/span>underscore<\/em> <\/span>variable has been from C convention.
    \nThe built-in library <\/span>gettext<\/code> <\/span>which is fori18n\/l10n <\/span><\/span>uses this convention, and Django which is Python web framework supports <\/span>i18n\/l10n<\/span> <\/span>also introduces and uses this convention.<\/p>\n
    # see official docs : https:\/\/docs.python.org\/3\/library\/gettext.html<\/a>\nimport gettext<\/pre>\n
    gettext.bindtextdomain('myapplication','\/path\/to\/my\/language\/directory')\ngettext.textdomain('myapplication')\n_ = gettext.gettext<\/pre>\n
    # ...<\/pre>\n
    print(_('This is a translatable string.'))<\/code><\/pre>\n
    To separate the digits of number literal value<\/h3>\n
    This feature was added in Python 3.6. It is used for separating digits of numbers using <\/span>underscore <\/span><\/em>for readability.<\/p>\n
    dec_base = 1_000_000\nbin_base = 0b_1111_0000\nhex_base = 0x_1234_abcd<\/pre>\n
    print(dec_base) # 1000000\nprint(bin_base) # 240\nprint(hex_base) # 305441741<\/pre>\n<\/blockquote>\n
    https:\/\/hackernoon.com\/understanding-the-underscore-of-python-309d1a029edc?gi=a08387cb7318<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"
    There are 5 cases for using the underscorein Python. For storing the value of last expression in interpreter. For ignoring the specific values. (so-called \u201cI don\u2019t care\u201d) To give special meanings and functions to name of vartiables or functions. To use<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"quote","meta":{"_mi_skip_tracking":false},"categories":[30,215],"tags":[47,222,1192],"jetpack_featured_media_url":"","jetpack_shortlink":"https:\/\/wp.me\/p8jhJx-sT","_links":{"self":[{"href":"https:\/\/rosetta.vn\/short\/wp-json\/wp\/v2\/posts\/1791"}],"collection":[{"href":"https:\/\/rosetta.vn\/short\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/rosetta.vn\/short\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/rosetta.vn\/short\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/rosetta.vn\/short\/wp-json\/wp\/v2\/comments?post=1791"}],"version-history":[{"count":0,"href":"https:\/\/rosetta.vn\/short\/wp-json\/wp\/v2\/posts\/1791\/revisions"}],"wp:attachment":[{"href":"https:\/\/rosetta.vn\/short\/wp-json\/wp\/v2\/media?parent=1791"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rosetta.vn\/short\/wp-json\/wp\/v2\/categories?post=1791"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rosetta.vn\/short\/wp-json\/wp\/v2\/tags?post=1791"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}