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Introduction to Python for beginners

Variables

>>> a = 4
>>> b = 6

>>> a + b
10

>>> a += 2
>>> a
6

>>> a = "Python"
>>> c = "MATLAB"
>>> z = " > "
>>> a + z + c
'Python > MATLAB'

Strings can have quotes or double quotes, there’s no difference in Python:

>>> first = 'a string'
>>> second = "b string"
>>> first + second
'a stringb string'

Length of a string:

>>> len(first)
8

Strings and numbers are different:

>>> number = "9"
>>> number + 6
Traceback (most recent call last):
   ...
TypeError: cannot concatenate 'str' and 'int' objects

We can convert between numbers and strings:

>>> int(number) + 6
15
>>> str(9)
'9'

However…

>>> number = "nine"
>>> int(number)
Traceback (most recent call last):
...
ValueError: invalid literal for int() with base 10: 'nine'

Lists

>>> an_empty_list = []
>>> list_with_two_items = [1, 2]
>>> items_can_be_diverse = [1, "Obama", 4.55]

>>> example_list = []
>>> example_list.append("experiment1")
>>> example_list
['experiment1']

>>> example_list[0]
'experiment1'

The length of a list (or any object that can have a length):

>>> len(example_list)
1

>>> example_list.append("failed_experiment")
>>> print(example_list)
['experiment1', 'failed_experiment']
>>> example_list.append("failed_experiment")
>>> print(example_list)
['experiment1', 'failed_experiment', 'failed_experiment']

>>> example_list.pop()
'failed_experiment'

>>> example_list
['experiment1', 'failed_experiment']

>>> del example_list[0]
>>> example_list
['failed_experiment']

range in Python 3 returns a “range object”. It’s like a list, but isn’t quite a list.

>>> range(10)
range(0, 10)

You can make it into a list by using the list constructor. A constructor is like a function, but it creates a new object, in this case a new object of type list.

>>> list(range(10))
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

You can also set the start element for range:

>>> list(range(2, 7))
[2, 3, 4, 5, 6]

Use in to ask if a element is a collection of things, such as a range, or a list:

>>> 5 in range(2, 7)
True
>>> 5 in [2, 5, 7]
True

>>> 9 in range(2, 7)
False

Sets

Sets are unordered, and unique.

“Unordered” means the order is arbitrary, and Python reserves the right to return the elements in any order it likes:

>>> our_work = set(["metacognition", "mindwandering", "perception"])
>>> print(our_work)  # doctest: +SKIP
{'mindwandering', 'perception', 'metacognition'}

If you want to get a version of the set that is ordered, use sorted, which returns a sorted list:

>>> sorted(our_work)
['metacognition', 'mindwandering', 'perception']

You can’t index a set, because the indices 0, or 1, or 2 don’t correspond to any particular element (because the set is unordered):

>>> our_work[0]
Traceback (most recent call last):
...
TypeError: 'set' object does not support indexing

Add to a set with the add method:

>>> our_work.add("consciousness")
>>> print(our_work)  # doctest: +SKIP
{'mindwandering', 'perception', 'metacognition', 'consciousness'}

>>> our_work.add("consciousness")
>>> print(our_work)  # doctest: +SKIP
{'mindwandering', 'perception', 'metacognition', 'consciousness'}
>>> our_work.add("consciousness")
>>> print(our_work)  # doctest: +SKIP
{'mindwandering', 'perception', 'metacognition', 'consciousness'}

You can subtract sets:

>>> competing_labs_work = set(["motor control", "decision making", "memory", "consciousness"])
>>> what_we_should_focus_on = our_work - competing_labs_work
>>> print(what_we_should_focus_on)  # doctest: +SKIP
{'mindwandering', 'perception', 'metacognition'}

>>> what_we_should_avoid = our_work.intersection(competing_labs_work)
>>> print(what_we_should_avoid)
{'consciousness'}

Sets have lengths as well:

>>> len(what_we_should_focus_on)
3

Strings

>>> example = "mary had a little lamb"
>>> print(example)
mary had a little lamb

String slicing:

>>> example[0]
'm'

>>> example[0:4]
'mary'

You can split strings with any character. This breaks up the string, returning a list of strings broken at the separator character:

>>> example.split(" ")
['mary', 'had', 'a', 'little', 'lamb']

>>> example.split(" ")[4]
'lamb'

You can split with any character:

>>> another_example = 'one:two:three'
>>> another_example.split(":")
['one', 'two', 'three']

You can also strip a string. That returns a new string with spaces, tabs and end of line characters removed from the beginning and end:

>>> my_string = ' a string\n'
>>> my_string
' a string\n'
>>> my_string.strip()
'a string'

Adding strings:

>>> example + " or two"
'mary had a little lamb or two'

Puting strings into other strings:

>>> subject_id = "sub1"
>>> print("Subject {} is excellent".format(subject_id))
Subject sub1 is excellent

>>> age = 29
>>> print("Subject {} is {} years old".format(subject_id, age))
Subject sub1 is 29 years old

You can do more complex formatting of numbers and strings using formatting options after a : in the placeholder for the string. See: https://docs.python.org/3.5/library/string.html#format-examples.

>>> print("Subject {:02d} is here".format(4))
Subject 04 is here

For loop

>>> for i in range(10):
...     print(i)
0
1
2
3
4
5
6
7
8
9

Identation is crucial!

>>> # for i in range(10):
>>> # print(i)

Watch out for mistakes:

>>> for i in range(10):
...     j = i + 1
>>> print(j)
10

Ifs and breaks

>>> a = 2
>>> b = 5
>>> c = a + b
>>> if c < 6:
...     print("yes")

>>> if c < 6:
...     print("yes")
... else:
...     print("no")
no

>>> if c < 6:
...     print("yes")
... elif c > 6:
...     print("no")
... else:
...     print("kind of")
no

>>> if True:
...     print("true, true!")
true, true!

>>> if False:
...     print("never!")

>>> for i in range(10):
...     if i == 6:
...         break
...     print(i)
0
1
2
3
4
5

>>> for i in range(10):
...     if i == 6:
...         continue
...     print(i)
0
1
2
3
4
5
7
8
9

Logic

You can use logical operators like and, or and not:

>>> strange_election = True
>>> uncomfortable_choices = True
>>> satisfying_experience = False
>>> strange_election and uncomfortable_choices
True
>>> strange_election and satisfying_experience
False
>>> strange_election and not satisfying_experience
True

We often use these in if statements:

>>> if strange_election and not satisfying_experience:
...     print('Watching a lot of news')
Watching a lot of news

Files

Write lines to a text file:

>>> fobj = open("/tmp/important_notes.txt", "wt")
>>> type(fobj)
<...>

>>> fobj.write("captains log 672828: I had a banana for breakfast.\n")
51
>>> fobj.write("captains log 672829: I should watch less TV.\n")
45
>>> fobj.close()

Read lines from a text file:

>>> fobj = open("/tmp/important_notes.txt", "rt")
>>> print(fobj.readline())
captains log 672828: I had a banana for breakfast.
<BLANKLINE>
>>> print(fobj.readline())
captains log 672829: I should watch less TV.
<BLANKLINE>

>>> print(fobj.readline())
<BLANKLINE>

Close a file when you’ve finished with it:

fobj.close()

One way to read all the lines from a text file:

>>> fobj = open("/tmp/important_notes.txt", "rt")
>>> lines = fobj.readlines()
>>> fobj.close()
>>> len(lines)
2
>>> print(lines[0])
captains log 672828: I had a banana for breakfast.
<BLANKLINE>
>>> print(lines[1])
captains log 672829: I should watch less TV.
<BLANKLINE>