Python Basic Syntax

Using Python as a calculator

Python can perform all the functionalities that advanced calculators can perform.

+ 1
* 2
** 2  # exponentiate

A successful execution in the REPL will often output the result of the computation and “print” the final results just after the code.

Later, some computations will produce results with multiple attributes, e.g. a model’s parameters, inputs, and runtime. What gets printed in these cases can vary widely so you shouldn’t assume you’ll be able to “view everything” when you’re running code. Instead you should/will learn to ask for what is available!

Python cares about indentation at the front of the line

One unique characteristic about Python is that its syntax forces us to write “neater” code. It enforces this by interpreting the empty spaces in front of the code as part of the command.

See which of the following will “break”

1+1
+ 1
+     1
+ 1

If you’re using iPython, you may not see an error in any of the commands above.

Reading error messages

Python’s error messages often have the word “Error” in them. This means that Python encountered a problem that forced the code execution to stop somewhere in the code but it may not be obvious what has and has not been executed.

Below we have two examples where we try to add the number 1 to the character "1", notice how the errors are different!

>>> 1 + "1"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'int' and 'str'
>>> 
>>> 
>>> 1 + 
  File "<stdin>", line 1
    1 + 
       ^
SyntaxError: invalid syntax

As a beginner, you should focus on:

the error type, e.g. “TypeError”. Knowing the category of error can quickly help us understand the issue in our code.
the error message, e.g. “unsupported operand type(s) for +: ‘int’ and ‘str’”
Python’s best guess at where the error occurred, e.g. line 1 will usually be the case for most REPL commands. This would be more helpful if your code were in a script.

Dealing with error messages

You should try to embrace error messages early on. Expert coders make errors all the time and the messages are really there to guide you instead of intimidate you.

The print() function

Similar to how calculators can perform log(1), Python has some built-in functions ready to use. One of the most useful ones is print().

It’s most useful when you’re not sure of the value of a variable. In particular, print() will accept numeric, character values, and more. This may seem trivial for now but later you will see that most functions are strict about the type of inputs it will accept.

Try running the code below line by line, then try to “copy&paste” blocks of the code. See how things may be different in a REPL.

print('hello world!')
x = 'hello universe!'
print(x)

x = 1.3
y = x * 3
z = x * y / 13
print(z)

print('hello', z)

Notice that our last print() function took 2 inputs that were separated by a comma.

Writing comments in code

Sometimes we write comments into our code to remind ourselves of some subtle choices. Comments will be ignored by Python but stay in the script as a descriptive reminder.

audience = 'class'
# The audience should come from the class list
print('hello {}'.format(audience))

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