Python provides the usual mix of arithmetic and comparison operators. However, there are some important wrinkles and features. Rather than assuming you're aware of them, we'll review the details.

The conventional arithmetic operators are: +, -, *, /, //, %, and **. There are two variations on division: an exact division (/) and an integer division (//). You must choose whether you want an exact, floating-point result, or an integer result:

>>> 355/113
3.1415929203539825
>>> 355//113
3
>>> 355.0/113.0
3.1415929203539825
>>> 355.0//113.0
3.0

The exact division (/) produces a float result from two integers. The integer division produces an integer result. When we use float values, we expect exact division to produce float. Even with two floating-point values, the integer division produces a rounded-down floating-point result.

We have this extra division operator to avoid having to use wordy constructs such as int(a/b) or math.floor(a/b).

Beyond conventional arithmetic, there are some additional bit fiddling operators that are available: &, |, ^, >>, <<, and ~. These operators work on integers (and sets). These are emphatically not Boolean operators; they don't work on the narrow domain of True and False. They work on the individual bits of an integer.

We'll use binary values with the 0b prefix to show what the operators do, as shown in the following code. We'll look at details of this 0b prefix later.

>>> bin(0b0101 & 0b0110)
'0b100'
>>> bin(0b0101 ^ 0b0110)
'0b11'
>>> bin(0b0101 | 0b0110)
'0b111'
>>> bin(~0b0101)
'-0b110'

The & operator does bitwise AND. The ^ operator does bitwise exclusive OR (XOR). The | operator does inclusive OR. The ~ operator is the complement of the bits. The result has many 1 bits and is shown as a negative number.

The << and >> operators are for doing left and right shifts of the bits, as shown in the following code:

>>> bin( 0b110 << 4 )
'0b1100000'
>>> bin( 0b1100000 >> 3 )
'0b1100'

It may not be obvious, but shifting left x bits is like multiplying it by 2**x, except it may operate faster. Similarly, shifting right by b bits amounts to division by 2**b.

We also have all of the usual comparison operators: <, <=, >, >=, ==, and !=.

In Python, we can combine comparison operators without including the AND operator:

>>> 7 <= 11 < 17
True
>>> 7 <= ll and 11 < 17
True

This simplification really does implement our conventional mathematical understanding of how comparisons can be written. We don't need to say 7 <= 11 and 11 < 17.

There's another comparison operator that's used in some specialized situations: is. The is operator will appear, for now, to be the same as ==. Try it. 3 is 3 and 3 == 3 seem to do the same thing. Later, when we start using the None object, we'll see the most common use for the is operator. For more advanced Python programming, there's a need to distinguish between two references to the same object (is) and two objects which claim to have the same value (==).

Python gives us a variety of numbers, plus the ability to easily add new kinds of numbers. We'll focus on the built-in numbers here. Adding new kinds of numbers is the sort of thing that takes up whole chapters in more advanced books.

Python ranks the numbers into a kind of tower. At the top are numbers with fewest features. Each subclass extends that number with more and more features. We'll look at the tower from bottom up, starting with the integers that have the most features, and moving towards the complex numbers that have the least features. The following sections cover the various kinds of numbers we'll need to use.

48813
0xbead 
0b1011111010101101
0o137255

>>> 2**256
115792089237316195423570985008687907853269984665640564039457584007913129639936

>>> from fractions import Fraction

>>> length=4+Fraction("7/8")
>>> width=2+Fraction("1/4")
>>> length*width
Fraction(351, 32)

>>> divmod(351,32)
(10, 31)

3.1415926
6.22E12

>>> import platform
>>> platform.python_build()
('v3.3.4:7ff62415e426', 'Feb  9 2014 00:29:34')
>>> platform.python_compiler()
'GCC 4.2.1 (Apple Inc. build 5666) (dot 3)'

>>> from decimal import Decimal

>>> conversion=Decimal("247.616")
>>> conversion
Decimal('247.616')

>>> lunch=Decimal("12900")
>>> lunch/conversion
Decimal('52.09679503747738433703799431')

>>> penny=Decimal('.00')
>>> (lunch/conversion).quantize(penny)
Decimal('52.10')
That's much better. How much was the bribe we needed to pay?
>>> bribe=50000
>>> (bribe/conversion).quantize(penny)
Decimal('201.93')

>>> cab=Decimal('23.50')
That gets us to the whole calculation: lunch plus bribe, converted, plus cab.
>>> ((lunch+bribe)/conversion).quantize(penny)+cab
Decimal('277.52')

>>> 2+3j
(2+3j)

Python includes a variety of data types, which aren't numbers. In the Handling text and strings section, we'll look at Python strings. We'll look at collections in Chapter 2, Acquiring Intelligence Data.

Boolean values, True and False, form their own little domain. We can extract a Boolean value from most objects using the bool() function. Here are some examples:

>>> bool(5)
True
>>> bool(0)
False
>>> bool('')
False
>>> bool(None)
False
>>> bool('word')
True

The general pattern is that most objects have a value True and a few exceptional objects have a value False. Empty collections, 0, and None have a value False. Boolean values have their own special operators: and, or, and not. These have an additional feature. Here's an example:

>>> True and 0
0
>>> False and 0
False

When we evaluate True and 0, both sides of the and operator are evaluated; the right-hand value was the result. But when we evaluated False and 0, only the left-hand side of and was evaluated. Since it was already False, there was no reason to evaluate the right-hand side.

The and and or operators are short-circuit operators. If the left side of and is False, that's sufficient and the right-hand side is ignored. If the left-hand side of or is True, that's sufficient and the right-hand side is ignored.

Python's rules for evaluation follow mathematic practice closely. Arithmetic operations have the highest priority. Comparison operators have a lower priority than arithmetic operations. The logical operators have a very low priority. This means that a+2 > b/3 or c==15 will be done in phases: first the arithmetic, then the comparison, and finally the logic.

Mathematical rules are followed by arithmetic rules. ** has a higher priority than *, /, //, or %. The + and – operators come next. When we write 2*3+4, the 2*3 operation must be performed first. The bit fiddling is even lower in priority. When you have a sequence of operations of the same priority (a+b+c), the computations are performed from left to right. If course, if there's any doubt, it's sensible to use parenthesis.

We've been using the REPL feature of our Python toolset. In the long run, this isn't ideal. We'll be much happier writing scripts. The point behind using a computer for intelligence gathering is to automate data collection. Our scripts will require assignment to variables. It will also require explicit output and input.

We've shown the simple, obvious assignment statement in several examples previously. Note that we don't declare variables in Python. We simply assign values to variables. If the variable doesn't exist, it gets created. If the variable does exist, the previous value is replaced.

Let's look at some more sophisticated technology for creating and changing variables. We have multiple assignment statements. The following code will assign values to several variables at once:

>>> length, width = 2+Fraction(1,4), 4+Fraction(7,8)
>>> length
Fraction(9, 4)
>>> width
Fraction(39, 8)
>>> length >= width
False

We've set two variables, length and width. However, we also made a small mistake. The length isn't the larger value; we've switched the values of length and width. We can swap them very simply using a multiple assignment statement as follows:

>>> length, width = width, length
>>> length
Fraction(39, 8)
>>> width
Fraction(9, 4)

This works because the right-hand side is computed in its entirety. In this case, it's really simple. Then all of the values are broken down and assigned to the named variables. Clearly, the number of values on the right have to match the number of variables on the left or this won't work.

We also have augmented assignment statements. These couple an arithmetic operator with the assignment statement. The following code is an example of +=: using assignment augmented with addition. Here's an example of computing a sum from various bits and pieces:

>>> total= 0
>>> total += (lunch/conversion).quantize(penny)
>>> total += (bribe/conversion).quantize(penny)
>>> total += cab
>>> total
Decimal('277.53')

We don't have to write total = total +.... Instead, we can simply write total += .... It's a nice clarification of what our intent is.

All of the arithmetic operators are available as augmented assignment statements. We might have a hard time finding a use for %= or **=, but the statements are part of the language.

The idea of a nice clarification should lead to some additional thinking. For example, the variable named conversion is a perfectly opaque name. Secrecy for data is one thing: we'll look at ways to encrypt data. Obscurity through shabby processing of that data often leads to a nightmarish mess. Maybe we should have called it something that defines more clearly what it means. We'll revisit this problem of obscurity in some examples later on.

Most of our missions will involve gathering and analyzing data. We won't be creating a very sophisticated User Interface (UI). Python has tools for building websites and complex graphical user interfaces (GUIs). The complexity of those topics leads to entire books to cover GUI and web development.

We don't want to type each individual Python statement at the >>> prompt. That makes it easy to learn Python, but our goal is to create programs. In GNU/Linux parlance, our Python application programs can be called scripts. This is because Python programs fit the definition for a scripting language.

For our purposes, we'll focus on scripts that use the command-line interface (CLI) Everything we'll write will run in a simple terminal window. The advantage of this approach is speed and simplicity. We can add graphic user interfaces later. Or we can expand the essential core of a small script into a web service, once it works.

What is an application or a script? A script is simply a plain text file. We can use any text editor to create this file. A word processor is rarely a good idea, since word processors aren't good at producing plain text files.

If we're not working from the >>> REPL prompt, we'll need to explicitly display the output. We'll display output from a script using the print() function.

Here's a simple script we can use to produce a receipt for bribing (encouraging) our informant.

From decimal import Decimal:

PENNY= Decimal('.00')

grd_usd= Decimal('247.616')
lunch_grd= Decimal('12900')
bribe_grd= 50000
cab_usd= Decimal('23.50')

lunch_usd= (lunch_grd/grd_usd).quantize(PENNY)
bribe_usd= (bribe_grd/grd_usd).quantize(PENNY)

print( "Lunch", lunch_grd, "GRD", lunch_usd, "USD" )
print( "Bribe", bribe_grd, "GRD", bribe_usd, "USD" )
print( "Cab", cab_usd, "USD" )
print( "Total", lunch_usd+bribe_usd+cab_usd, "USD" )

Let's break this script down so that we can follow it. Reading a script is a lot like putting a tail on an informant. We want to see where the script goes and what it does.

First, we imported the Decimal definition. This is essential for working with currency. We defined a value, PENNY, that we'll use to round off currency calculations to the nearest penny. We used a name in all caps to make this variable distinctive. It's not an ordinary variable; we should never see it on the left-hand side of an assignment statement again in the script.

We created the currency conversion factor, and named it grd_usd. That's a name that seems meaningful than conversion in this context. Note that we also added a small suffix to our amount names. We used names such as lunch_grd, bribe_grd, and cab_usd to emphasize which currency is being used. This can help prevent head-scrambling problems.

Given the grd_usd conversion factor, we created two more variables, lunch_usd and bribe_usd, with the amounts converted to dollars and rounded to the nearest penny. If the accountants want to fiddle with the conversion factor—perhaps they can use a different bank than us spies—they can tweak the number and prepare a different receipt.

The final step was to use the print() function to write the receipt. We printed the three items we spent money on, showing the amounts in GRD and USD. We also computed the total. This will help the accountants to properly reimburse us for the mission.

We'll describe the output as primitive but acceptable. After all, they're only accountants. We'll look into pretty formatting separately.

The simplest way to gather input is to copy and paste it into the script. That's what we did previously. We pasted the Greek Drachma conversion into the script: grd_usd= Decimal('247.616'). We could annotate this with a comment to help the accountants make any changes.

Additional comments come at the end of the line, after a # sign. They look like this:

grd_usd= Decimal('247.616') # Conversion from Mihalis Bank 5/15/14

This extra text is part of the application, but it doesn't actually do anything. It's a note to ourselves, our accountants, our handler, or the person who takes over our assignments when we disappear.

This kind of data line is easy to edit. But sometimes the people we work with want more flexibility. In that case, we can gather this value as input from a person. For this, we'll use the input() function.

We often break user input down into two steps like this:

        entry= input("GRD conversion: ")
        grd_usd= Decimal(entry)

The first line will write a prompt and wait for the user to enter the amount. The amount will be a string of characters, assigned to the variable entry. Python can't use the characters directly in arithmetic statements, so we need to explicitly convert them to a useful numeric type.

The second line will try to convert the user's input to a useful Decimal object. We have to emphasize the try part of this. If the user doesn't enter a string that represents valid Decimal number, there will be a major crisis. Try it.

The crisis will look like this:

>>> entry= input("GRD conversion: ")
GRD conversion: 123.%$6
>>> grd_usd= Decimal(entry)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
decimal.InvalidOperation: [<class 'decimal.ConversionSyntax'>]

Rather than this, enter a good number. We entered 123.%$6.

The bletch starting with Traceback indicates that Python raised an exception. A crisis in Python always results in an exception being raised. Python defines a variety of exceptions to make it possible for us to write scripts that deal with these kinds of crises.

Once we've seen how to deal with crises, we can look at string data and some simple clean-up steps that can make the user's life a little easier. We can't fix their mistakes, but we can handle a few common problems that stem from trying to type numbers on a keyboard.

    entry= input("GRD conversion: ")
    try:
        grd_usd= Decimal(entry)
    except decimal.InvalidOperation:
        print("Invalid: ", entry)

grd_usd= None
while grd_usd is None:
    entry= input("GRD conversion: ")
    try:
        grd_usd= Decimal(entry)
    except decimal.InvalidOperation:
        print("Invalid: ", entry)
print( grd_usd, "GRD = 1 USD" )