Study notes about Python during access webdata

”-“ as a variable

_ has 3 main conventional uses in Python:

• To hold the result of the last executed statement in an interactive interpreter session. This precedent was set by the standard CPython interpreter, and other interpreters have followed suit

• For translation lookup in i18n (see the gettext documentation for example), as in code like: raise forms.ValidationError(_(“Please enter a correct username”))

• As a general purpose “throwaway” variable name to indicate that part of a function result is being deliberately ignored, as in code like:

label, has_label, _ = text.partition(':')

• The latter two purposes can conflict, so it is necessary to avoid using _ as a throwaway variable in any code block that also uses it for i18n translation (many folks prefer a double-underscore, __, as their throwaway variable for exactly this reason).

<a> tag

• The a tagtag defines a hyperlink, which is used to link from one page to another.

• The most important attribute of the <a> element is the href attribute, which indicates the link’s destination.

• By default, links will appear as follows in all browsers:

  • An unvisited link is underlined and blue

  • A visited link is underlined and purple

  • An active link is underlined and red

href Attribute

• For <a> and <area> elements, the href attribute specifies the URL of the page the link goes to.

• For <base> elements, the href attribute specifies the base URL for all relative URLs on a page.

• For <link> elements, the href attribute specifies the location (URL) of the external resource (most often a style sheet file).

Attributes

• All HTML elements can have attributes

• Attributes provide additional information about an element

• Attributes are always specified in the start tag

• Attributes usually come in name/value pairs like: name=”value”

tag.get(key, None) how to understand None

• which will return None if key is not in tag. You can also provide a different default value that will be returned instead of None:

 value = tag.get(key, "empty")   

Names of variable

• The Rules

  • Variables names must start with a letter or an underscore,

    • _underscore

    • underscore_

  • The remainder of your variable name may consist of letters, numbers and underscores.

    • password1

    • n00b. …

    • un_der_scores

  • Names are case sensitive.

    • case_sensitive, CASE_SENSITIVE, and Case_Sensitive are each a different variable.

• The Conventions

  • Readability is very important. Which of the following is easiest to read? I’m hoping you’ll say the first example.

    • python_puppet

    • pythonpuppet

    • pythonPuppet

  • Descriptive names are very useful. If you are writing a program that adds up all of the bad puns made in this book, which do you think is the better variable name?

    • total_bad_puns

    • super_bad

  • Avoid using the lowercase letter ‘l’, uppercase ‘O’, and uppercase ‘I’. Why? Because the l and the I look a lot like each other and the number 1. And O looks a lot like 0.