Learning Python – Part 1

Forewords

A while back I decided to try and learn Python for the hell of it as it seems like an interesting language and has some of the most concise and user-friendly syntax. Having spent some time going through a number of different learning sources and materials (like the official site python.org which has a very helpful tutorial section) I have put together a set of notes I made as I was learning and hopefully they can be useful to you as a quick list of how-to code snippets.

All the code snapshots I’m showing here are taken from the IDLE Python shell.

Basics

Comments:

 

Variable assignment:

 

Arithmetic:

 

Power:

 

Absolute value:

 

Getting user input:

 

raw_input vs input:

raw_input always contains string, input can contain any object, even a calculation:

 

Import modules:

 

Functions as first class objects:

 

If-elseif-else:

 

The is operator checks if two variables refers to the SAME object:

on the other hand:

The is not operator does the reverse.

 

The and and or logical operators, same as && and || in C# respectively. You can use the not operator to negate the outcome of a boolean comparison.

 

You can chain comparisons, e.g. is the value of x greater than or equal to 5 and less than or equal to 10?

 

You may compare sequence objects of the same type, which uses lexicographical ordering – compare the first two, and if they differ then that’s the outcome of the comparison, else compare the next two, and so on:

 

Strings

Strings can use double or single quotes interchangeably:

Escape character:

Spanning across multiple lines – a backslash (\) as the last character on the line indicates that the next time is a logical continuation of this line:

or you can surround them in a pair of matching triple quotes: """ or ”’:

 

String conversion using the str() function:

The repr function – the repr function returns a canonical string representation of the object, back-ticks (`) do the same thing (they are similar to the ToString() method on C#’s objects:

 

String concatenation:

 

Slicing a string:

You can also use negative index, in which case it starts counting from the right:

note: message[0] = message[-0], see how the indices are mapped:

you can also set up steps in the slicing:

similarly to before, you can slice backwards too:

 

Get length of string:

 

Strings are IMMUTABLE!

 

Formatting strings:

 

Finding substring (returns the index of the start of the first match):

 

Joining strings:

 

Changing the case of strings:

 

Replacing portions of a string: