Variables

In the heart of every good programming language, including Python, are the variables. Variables are an awesome asset to the dynamic world of the web. Variables alone are dynamic by nature. However, Python is pretty smart when it comes to variables, but it is sometimes quite a pain. Python interprets and declares variables for you when you set them equal to a value. Example:

Example
a = 0
b = 2
print(a  + b)
Result

2

Isn’t that cool how Python figured out how a and b were both numbers. Now, let’s try it with the mindset of wanting the 0 to be a string and the 2 to also be a string to create a new string “02”.

Example
a = "0"
b = "2"
print(a  + b)
Result

02

Ah, see! Now, it thinks they are both strings simply because we put them in quotations. Don’t worry if you don’t understand strings yet, just note that they are not numbers or integers. This is really awesome, but as with any element or practice in a programming language there is always the flip side, especially with this auto declaration thing. Suppose we started off with “0” being a string, but we change our mind and want it to be a number. Finally, we decide that we want to add it to the number 2. Python bites us with an error. This brings us to the idea of casting the variable into a certain type.

Example
a = "0"
b = 2
print(int(a)  + b)
Result

2

Whoa, what is that int() thing? This is how you cast one variable type into another. In this example, we cast the string 0 to an integer 0. Let’s take a quick peak at a few of the important variable types in Python:

  • int(variable) – casts variable to integer
  • str(variable) – casts variable to string
  • float(variable) – casts variable to float (number with decimal)

Like I said, those are just the most commonly used types of casting. Most of the other variable types you typically wouldn’t want to cast to.

Operators

With every programming language we have our operators, and Python is no different. Operators in a programming language are used to vaguely describe 5 different areas: arithmetic, assignment, increment/decrement, comparison, logical. There isn’t really much need to explain all of them as I have previously in the other categories, which means we will only cover a few of them.

Arithmetic Operators

Example
print (3 + 4)
print (3 - 4)
print (3 * 4)
print (3 / 4)
print (3 % 2)
print (3 ** 4) # 3 to the fourth power
print (3 // 4) #floor division
Result

7
-1
12
0.75
1
81
0

See, they are pretty standard. I included how to do exponents because it’s a little funky, but other than that, they are fairly normal and work as expected. Don’t forget that you can use the + sign to concatenate strings. The addition, subtraction, multiplication, and division work just like expected. You might have not seen the modulus operator before (%). All the modulus operator does is to divide the left side by the right side and get the remainder. So, that remainder is what is returned and not the number of times the right number went into the left number The double * is just an easy way to provide exponents to Python. Finally, the floor division operator (//) just divides the number and rounds down.

Assignment Operators

These are pretty identical to the previous languages also, but a refresher never hurt anyone. Example please!

Example
a = 0
a+=2
print (a)
Result

2

Of course, you can do this with any of the previous arithmetic operators as an assignment operator followed by the = We just told Python to add 2 to the value of a without having to say something like a = a + 2. We are programmers, and we are proud to be called lazy!

If Statements

In the heart of programming logic, we have the if statement in Python. The if statement is a conditional that, when it is satisfied, activates some part of code. Often partnered with the if statement are else if and else. However, Python’s else if is shortened into elif. If statements are generally coupled with variables to produce more dynamic content. Let’s cut to an example really quick.

Example
a = 20
if a >= 22:
    print("if")
elif a >= 21:
    print("elif")
else:
    print("else")
Result

else

So, we have the variable a that equals twenty. Now, we run it through our if statement that checks to see if a is greater than or equal to 22. It isn’t. So, we skip past the inner print statement and continue to the elif statement. This conditional checks if a is greater than or equal to 21. It isn’t either, which brings us to the final leg of our expanded if statement. The else is the catch all, which means if the previous conditionals are not satisfied, we will run the code inside it. So, we run the print(“else”), and we see in the results, the string else.

The If Syntax

The most important thing you might have missed in the example above is how Python’s syntax is a lot cleaner than other languages. However, this also means it is very picky and tends to bite beginners with errors. After every conditional we have a colon. Next, you must proceed to a new line with 4 spaces to tell Python you only want this code with 4 spaces to be run when the previous conditional is satisfied. Ok, well it doesn’t have to be 4 spaces, but you must be consistent with the spaces you use to indent. You can use 3 every time if you want, but 4 is kind of a standard. Also, if you wanted to run more than one statement after the conditional is satisfied, you must have a new line with the same number of spaces before the next statement.

Functions

Functions in Python are super useful in separating your code, but they don’t stop there. Any code that you think you will ever use anywhere else, you should probably put in a function. You can also add arguments to your functions to have more flexible functions, but we will get to that in a little bit. You can define a function by using the keyword def before your function name. Let’s use our first Python function.

Example
def someFunction():
    print("boo")
someFunction()
Result

boo

It is necessary to define functions before they are called. Even though we have to skip over the function while reading to see the first statement, someFunction(). This throws us back up into the def someFunction():, again with a colon following it. Then after we acknowledge that the function is being called, we create a new line with four spaces for our simple print statement.

Functions with Arguments

Simple functions like the one above are great and can be used quite often. However, there usually comes a time where we want to have the function act on data that the user inputs. We can do this with arguments inside of the () the follows the function name.

Example
def someFunction(a, b):
    print(a+b)
someFunction(12,451)
Result

463

Using the statement someFunction(12,451), we pass in 12, which becomes a in our function, and 451, which becomes b. Then, we just have a little print statement that adds them and prints them out.

Function Scope

Python does support global variables without you having to explicitly express that they are global variables. It’s much easier just to show rather than explain:

Example
def someFunction():
    a = 10
someFunction()
print (a)

This will cause an error because our variable, a, is in the local scope of someFunction. So, when we try to print a, Python will bite and say a isn’t defined. Technically, it is defined, but just not in the global scope. Now, let’s look at an example that works.

Example
a = 10 
def someFunction():
    print (a)
someFunction()

In this example, we defined a in the global scope. This means that we can call it or edit it from anywhere, including inside functions. However, you cannot declare a variable inside a function, local scope, to be used outside in a global scope.

For Loop

It’s time for an awesome part of Python. Python’s for loops are pretty amazing compared to some other languages because of how versatile and simple they are. The idea of a for loop is rather simple, you will just loop through some code for a certain number of times. I won’t get a chance to show you the flexibility of the for loops until we get into lists, but sure enough its time will come. Onto an example:

Example
for a in range(1,3):
    print (a)
Result

1
2

First, print is indented for a reason. Remember that Python is picky about spacing. It is somewhat complicated to understand exactly what a is in the example. In this instance, a is a variable the increments itself every time we run through the loop. Next, we use the range keyword to set the starting point and the point right after we would finish. This is precisely why the number 3 didn’t print. Python is quite fond of this idea of all the way up to a number, but not including it.

Also, notice the in keyword. This is actually part of the for loop and you will understand it a bit better after we deal with lists and dictionaries. So, basically the above for loop says, “For variable a, which will be incremented at the end of every loop, in the range of 1 up to 3.”