12: Tips

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This lesson contains a few facts about the Python language which we left out of earlier lessons.

print Options

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.

Example

Examples of sep and end. Look at the effect of each print call.

print('D', 'A', 'S', 'H', sep='-') print('The price is $', 12*5, sep='') print('These two lines', end='') print(' will appear together.') print('Great', end='!')

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.

Coding Exercise: Alphabet Aerobics

Using what we just introduced, debug the following program so that it outputs the upper-case alphabet all on one line.

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for c in range(0, 26): print(chr(ord('A')+c))

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Valid Variable/Function Names

• Names can contain letters, numbers, and the underscore (_) character.
• The first character of every name must be a letter.
• Python distinguishes between upper- and lower-case letters.

Thus my_3rd_int is a valid variable name, but 3rd_int is not.

Example

An example of distinguishing between upper- and lower-case.

greaterOde = "Truth is beauty, beauty truth" greatErode = "Wind erosion" print(greaterOde) # still poetry

Descending for loops & other increments

Recall that we showed how for loops can iterate through numbers in increasing order:

Example

A for loop

for i in range(0, 5): print(i)

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:

Example

Negative step for range

for i in range(5, 1, -1): print(i)

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.

Coding Exercise: Lucky Sevens

Write a program, using a for loop with step size 10, to print out all 2-digit numbers which end in 7, in increasing order.

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delete this comment and enter your code here

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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 y > x).

With a for loop, another way to accomplish a decreasing range is reversed(range(x, y)), which goes from y-1 to x in decreasing order.

Writing Smaller Code

Python allows several ways to write shorter code; we introduce a few of them here.

Assignment Operators

Python lets you write "x += 1" to mean "add one to x." So it is equivalent to "x = x + 1" as we show below:

Example

Examples of +=, -=, *=, /=

x = 8 x += 2 print(x) x -= 1 print(x) x /= 3 print(x) x *= 10 print(x)

Similarly, there are operators for integer division (//=), modulus (%=), and power (**=).

Inline Blocks

We have seen several statements which are followed by indented "blocks" of code: for, if, else, elif, while, and def, for example:

if x==y:
  «block» #indented, multiline

In the special case that the «block» is only one line long, Python allows the following alternative syntax:

if x==y: «block» #single line

Here is an example:

Example

Inline block statements

def square(x): return x*x for i in range(0, 6): print(square(i))

This has its limits however: a compound block with a colon (if, for, etc) can't be used as an inline block. For example, if 2>1: if 4>3: print() gives a syntax error.

Multiple Assignments

Python allows you to combine two assignment statements into one:

Example

Multiple assignment

x, y = 3, 4 print('x =', x, 'and y =', y)

Note that this allows you to solve our swapping exercise using only one line!

Comparison Chaining

Python also allows you to combine several comparisons into one:\

Example

Multiple assignment

age = int(input()) if 12 < age < 20: # same as (12 < age) and (age < 20) print('You are a teenager!')

Any chain v1 c1 v2 c2 v3 ... where v are values and c are comparisons, is treated the same as (v1 c1 v2) and (v2 c2 v3) and...

Default values for [:] and range()

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 range(0, n).

Example

Using the default values for [:] and range()

S = "Toronto" print(S[3:]) # right substring, "onto" print(S[:2]) # left substring, "To" for i in range(5): print(i*i) # prints 0, 1, .. 16

None

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:

Example

The value of print

result = print('Anything works here') print(result) def forgotToReturn(x): x*x print(forgotToReturn(5))

Here 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:

Example

The type of a type

x = 23432 print(x, type(x), type(type(x)), type(type(type(x))))

In the next exercise, we ask you to carefully track the results of a long command using type and print.

Scramble Exercise: One None

The output of print(type(print(type(type(print(print())))))) consists of 4 lines. Put them in the correct order.

• None
• «blank line»

Correct!

That is all of the tips for now. You are ready to continue to the next lesson!