1.2. Learn Python

In case you have not written a single line of Python before, the internet offers a great assortment of resources to learn Python. Here are some links:

• Python 3.5 tutorial from the official site
• Python 3.5 API documentation
• LearnPython (Python 2)
• Gallery of Notebooks (see, e.g. Scientific Computing with Python)

Note

When reading tutorials from the internet, keep in mind that they might be written in Python 2. While most things work the same in Python 3, there are some differences. One quick check is to look at the code, if it calls print without parentheses (print "foo"), then it is Python 2, in Python 3 the parentheses are required (print("foo")). Some of the key differences are explained, e.g. in a blog post by Sebastian Raschka.

Some basic syntax examples (note that Python is a dynamic language, which allows mixing of different object types in lists, etc.):

>>> l = [] # make an empty list

>>> l = ['a', 1] # make a list with two elements

>>> l.append('b') # add an element to list

>>> print(l)
['a', 1, 'b']

>>> l[0] # returns the first element of the iterable (list)
'a'

>>> l[-1] # returns the last element of the iterable (list)
['b']

>>> l[1:] # everything but the first element (slicing)
[1, 'b']

>>> l[:-1] # everything but the last element
['a', 1]

>>> l = l + ['c', 3, 4] # concatenate two lists

>>> t = ('c', 2) # make a tuple, "an immutable list"

>>> t = tuple(l) # return a copy of the list as a tuple

>>> l = list(t) # return a copy of the tuple as a list

>>> d = {} # Make an empty dictionary

>>> d['foo'] = 'bar' # assign value 'bar' to key 'foo'

>>> d[1] = 2 # dictionaries can have any hashable object as a key

>>> d[[1, 2, 3]] = 3 # lists are not hashable
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'

>>> d[(1, 2, 3)] = 3 # but tuples are

>>> if 1 in d: # test if 1 is in the keys of the dictionary 'd'
...     print('Found it!')
Found it!

>>> for key, value in d.items(): # loop over key-value pairs
...     print(key, value)
1 2
foo bar

>>> for i in range(3):
...     print(i)
0
1
2

>>> l = [(i, chr(i)) for i in range(97, 123)] # create a list with for loop

>>> def awesome(you): # function definition
...     print("You are awesome, {}!".format(you))

>>> def unfathomable(who='Cthulhu'): # default parameter value for 'who'
...     return "{} is unfathomable!".format(who)

>>> unfathomable()
'Cthulhu is unfathomable!'