This lesson contains a few facts about the Python language which we left out of earlier lessons.
So far, the only way we have seen to print is to use the
print() function, which prints several items one line at a time, and separates all items by spaces. In fact, print accepts a few keyword arguments which allow you to modify this behaviour to suit your needs.
- To change the space separator to some other string
S, include the argument
sep = «S»
- To replace the "end-of-line" character with some other string
S, include the argument
end = «S»
This new feature, keyword arguments, is easiest to demonstrate with a few examples.
These keyword arguments must be passed at the end of the argument list or else an error will occur. If you use both they can be given in any order.
Valid Variable/Function Names
- Names can contain letters, numbers, and the underscore (
- The first character of every name must be a letter.
- Python distinguishes between upper- and lower-case letters.
my_3rd_int is a valid variable name, but
3rd_int is not.
for loops & other increments
Recall that we showed how
for loops can iterate through numbers in increasing order:
Quite often, it is necessary to write a
for loop which goes through the numbers in descending order (biggest to smallest). To do this we call
range with a third argument called the step:
If you think for a moment, you also notice
range(0, 5) is the same as
range(0, 5, 1): the default increment is 1. In either case, be careful that
range(start, stop, step) goes until just before reaching the value
stop. For more information you can see the Python manual.
Similarly, you can use
string[x:y:2] to get the substring with characters
x, x+2, x+4, ... or
string[y:x:-1] to get a reversed part of a string (where
for loop, another way to accomplish a decreasing range is
reversed(range(x, y)), which goes from
x in decreasing order.
Writing Smaller Code
Python allows several ways to write shorter code; we introduce a few of them here.
|On this website we won't always use these features, since sometimes it makes code harder to read.|
Python lets you write "
x += 1" to mean "add one to
x." So it is equivalent to "
x = x + 1" as we show below:
Similarly, there are operators for integer division (
//=), modulus (
%=), and power (
We have seen several statements which are followed by indented "blocks" of code:
def, for example:
if x==y: «block» #indented, multilineIn the special case that the
«block»is only one line long, Python allows the following alternative syntax:
if x==y: «block» #single lineHere is an example:
This has its limits however: a compound block with a colon (
for, etc) can't be used as an inline block. For example,
if 2>1: if 4>3: print() gives a syntax error.
Python allows you to combine two assignment statements into one:
Note that this allows you to solve our swapping exercise using only one line!
Python also allows you to combine several comparisons into one:
v1 c1 v2 c2 v3 ...where
vare values and
care comparisons, is treated the same as
(v1 c1 v2) and (v2 c2 v3) and...
Default values for
You can leave out one or both of the start/end values when using the sub-string operator
[:]. (This is also true for lists, as we'll see in the next lesson.) The default value of the first index is
0 (the start of the string) and the default value of the last index is the length of the string. Likewise,
range(n) is short for
Some functions return a value as their main effect, like
abs(x). But other functions which are more valuable for their effects, like
print(), still return the 'default' value
None is a special value used by Python as a general-purpose placeholder. If you call type(None), you find out that
None even has its own type,
NoneType. While we are at it, let's see what is the type of a type variable:
In the next exercise, we ask you to carefully track the results of a long command using
print(type(print(type(type(print(print()))))))consists of 4 lines. Put them in the correct order.
- <class 'type'>
- «blank line»
- <class 'NoneType'>
That is all of the tips for now. You are ready to continue to the next lesson!