$ python >>> help(str)For now, ignore methods with names that begin with __. If you want to know the full gory details, see the Technical Discussion at the end of this document. There are a lot of additional string methods not beginning with __ that you can now use. The example program, stringOps.py, has some things you can try out. Let me know if you have any questions about what it is doing:
""" A program showing some string operations using the dot notation Author: Andrew Danner Date: October 2008 """ def main(): """ many of these one-liners can be typed directly into the python shell for more practice. """ s="hello" print "s = ",s print "s.upper() = ", s.upper() print "s.isalpha() = ", s.isalpha() print numStr="1992" # Last Pittsburgh Pirate winning season print "numStr = ", numStr print "numStr.isalpha() = ", numStr.isalpha() print "numStr.isdigit() = ", numStr.isdigit() print sentence="Check it out!" print "\n\nsentence = ", sentence print "sentence.isalpha() = ", sentence.isalpha() words = sentence.split() print "sentence.split() = ", words print # use join to undo a split # syntax is: <string>.join(<list of strings>) # semantics: join all elements in <list of strings> together with # <string> in between each element separator='' s2 = separator.join(ls) # convert from list of characters to string print "s2 = ",s2 main()
Another new syntactic feature is the if <item> in <list>:. This statement evaluates to True and executes the body of the if statement if the specified item is present one or more times in the given list.
List elements can be modified. The syntax l=x changes the contents of the first item in the list l to now contain the element x. Even though strings support index and slicing syntax (e.g., s, s[1:3], strings cannot be modified in this way. You can however convert a string to a list of characters, modify the list and then convert the list back to a string. The end of the program shows an example of this. Could this be how the method s.replace(old, new) works in the string class?
For more on list methods, type help(list) in a python shell. Again, ignore methods that begin with the __ prefix which are invoked using using an alternate syntax. In listOps.py are some examples of list methods. We will talk about some of these methods later in the course as we need them.
""" A program showing some list operations using the dot notation Author: Andrew Danner Date: October 2008 """ def main(): """ many of these one-liners can be typed directly into the python shell for more practice. look at help(list) for more python list operations """ l=[1,5,4] print "l = ",l l.append(6) # append modifies l. Returns nothing print "l after l.append(6) = ", l l.append(8) print "l after l.append(8) = ", l if 6 in l: print "yes, 6 is in l" s="hello" ls=list(s) # create a list of characters from s print "s=",s," ls = ", ls #s='j' # illegal, strings cannot be modified in place ls='j' # ok, lists can be modified print "modified ls = ", ls s2=''.join(ls) # convert from list of characters to string print "s2 = ",s2 main()
Methods with names that begin with __ (like __add__) are special methods that usually have a simpler syntax that is cleaner than using method names. You probably already know the shortcuts, but for those that want to know more, you can also call these method names directly using x.__methodname__(argument) (where x is a string and the argument's type is appropriate for the given method). Using the alternate notation shown in the help documentation is the prefered way to use these methods. For example, for string concatenation use x + y instead of x.__add__(y). You can see that we have been using some of these special methods for quite awhile:
s = "hello" y = s + " there" # prefered syntax for string concatenation y = s.__add__(" there") # this does the same thing, but don't use this notation print len(s) # len(s) is also s.__len__()Basically, when python sees x+y, it automatically rewrites the code as x.__add__(y). As programmers, the x+y syntax is usually cleaner, more natural, easier to write, and easier to understand.
For lists, the __ methods work similarly, e.g.,
l1 = [1, 2, 3, 17] l2 = l1[2:4] # prefered syntax for the slice operation l2 = l1.__getslice__(2,4) # this does the same thing, but has icky syntax