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There are about 130 names in the list, so we have omitted most of them Those that begin with a capital letter are the names of Python s built-in exceptions; the rest are function and data type names The second convention concerns the use of underscores (_) Names that begin and end with two underscores (such as __lt__) should not be used Python de nes various special methods and variables that use such names (and in the case of special methods, we can reimplement them, that is, make our own versions of them), but we should not introduce new names of this kind ourselves We will cover such names in 6 Names that begin with one or two leading underscores (and that don t end with two underscores) are treated specially in some contexts We will show when to use names with a single leading underscore in 5, and when to use those with two leading underscores in 6 A single underscore on its own can be used as an identi er, and inside an interactive interpreter or Python Shell, _ holds the result of the last expression that was evaluated In a normal running program no _ exists, unless we use it explicitly in our code Some programmers like to use _ in for in loops when they don t care about the items being looped over For example:
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for _ in (0, 1, 2, 3, 4, 5): print("Hello")
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Be aware, however, that those who write programs that are internationalized often use _ as the name of their translation function They do this so that instead of writing gettextgettext("Translate me"), they can write _("Translate me") (For this code to work we must have rst imported the gettext module so that we can access the module s gettext() function) Let s look at some valid identi ers in a snippet of code written by a Spanishspeaking programmer The code assumes we have done import math and that the variables radio and vieja_ rea have been created earlier in the program: = mathpi = 00000001
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nueva_ rea = * radio * radio if abs(nueva_ rea - vieja_ rea) < : print("las reas han convergido")
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We ve used the math module, set epsilon ( ) to be a very small oating-point number, and used the abs() function to get the absolute value of the difference between the areas we cover all of these later in this chapter What we are concerned with here is that we are free to use accented characters and Greek letters for identi ers We could just as easily create identi ers using Arabic, Chinese, Hebrew, Japanese, and Russian characters, or indeed characters from any other language supported by the Unicode character set
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The easiest way to check whether something is a valid identi er is to try to assign to it in an interactive Python interpreter or in IDLE s Python Shell window Here are some examples:
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When an invalid identi er is used it causes a SyntaxError exception to be raised In each case the part of the error message that appears in parentheses varies, so we have replaced it with an ellipsis The rst assignment fails because - is not a Unicode letter, digit, or underscore The second one fails because the start character is not a Unicode letter or underscore; only continuation characters can be digits No exception is raised if we create an identi er that is valid even if the identi er is the name of a built-in data type, exception, or function so the third assignment works, although it is ill-advised The fourth fails because a quote is not a Unicode letter, digit, or underscore The fth is ne
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