第116页 | Learning Python | 阅读 ‧ 电子书库

Getting Help

The methods introduced in the prior section are a representative, but small, sample of what is available for string objects. In general, this book is not exhaustive in its look at object methods. For more details, you can always call the built-in dir function, which returns a list of all the attributes available for a given object. Because methods are function attributes, they will show up in this list. Assuming S is still the string, here are its attributes on Python 3.0 (Python 2.6 varies slightly):

>>> dir(S)
['__add__', '__class__', '__contains__', '__delattr__', '__doc__', '__eq__',
'__format__', '__ge__', '__getattribute__', '__getitem__', '__getnewargs__',
'__gt__', '__hash__', '__init__', '__iter__', '__le__', '__len__', '__lt__',
'__mod__', '__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__',
'__repr__', '__rmod__', '__rmul__', '__setattr__', '__sizeof__', '__str__',
'__subclasshook__', '_formatter_field_name_split', '_formatter_parser',
'capitalize', 'center', 'count', 'encode', 'endswith', 'expandtabs', 'find',
'format', 'index', 'isalnum','isalpha', 'isdecimal', 'isdigit', 'isidentifier',
'islower', 'isnumeric', 'isprintable', 'isspace', 'istitle', 'isupper', 'join',
'ljust', 'lower', 'lstrip', 'maketrans', 'partition', 'replace', 'rfind',
'rindex', 'rjust', 'rpartition', 'rsplit', 'rstrip', 'split', 'splitlines',
'startswith', 'strip', 'swapcase', 'title', 'translate', 'upper', 'zfill']

You probably won’t care about the names with underscores in this list until later in the book, when we study operator overloading in classes—they represent the implementation of the string object and are available to support customization. In general, leading and trailing double underscores is the naming pattern Python uses for implementation details. The names without the underscores in this list are the callable methods on string objects.

The dir function simply gives the methods’ names. To ask what they do, you can pass them to the help function:

>>> help(S.replace)
Help on built-in function replace:

replace(...)
    S.replace (old, new[, count]) -> str

    Return a copy of S with all occurrences of substring
    old replaced by new.  If the optional argument count is
    given, only the first count occurrences are replaced.

help is one of a handful of interfaces to a system of code that ships with Python known as PyDoc—a tool for extracting documentation from objects. Later in the book, you’ll see that PyDoc can also render its reports in HTML format.

You can also ask for help on an entire string (e.g., help(S)), but you may get more help than you want to see—i.e., information about every string method. It’s generally better to ask about a specific method.

For more details, you can also consult Python’s standard library reference manual or commercially published reference books, but dir and help are the first line of documentation in Python.