Introduction to Python#
Variables#
Variables are named values.
a = 4
Read this as:
“a” gets the value 4.
b = 6
“b” gets the value 6.
a + b
10
You can also update variables, as in:
a = a + 2
a
6
So far we’ve seen numbers as values, but we can also have bits of text called strings. Strings are bits of text between quotes.
a = "Python"
c = "MATLAB"
z = " > "
You have seen adding numbers. We can also add strings. This as the effect of sticking them together — concatenating the strings:
a + z + c
'Python > MATLAB'
Strings can have apostrophes ('
) or inverted commas ("
) at either end,
Python accepts either.
first = 'a string'
second = "b string"
first + second
'a stringb string'
You can even use triple apostrophes or inverted commas at either end; this is useful for when you want to put a new line inside your string:
many_line_string = """
This string
has
several
lines
"""
print(many_line_string)
This string
has
several
lines
Length of a string:
len(first)
8
Strings and numbers are different:
number = "9"
number + 6
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
Cell In[10], line 2
1 number = "9"
----> 2 number + 6
TypeError: can only concatenate str (not "int") to str
We can convert between numbers and strings:
int(number) + 6
str(9)
'9'
However …
number = "nine"
int(number)
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
Cell In[12], line 2
1 number = "nine"
----> 2 int(number)
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)
example_list.append("failed_experiment")
print(example_list)
['experiment1', 'failed_experiment']
['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 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)
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)
{'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]
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
Cell In[28], line 1
----> 1 our_work[0]
TypeError: 'set' object is not subscriptable
Add to a set with the add
method:
our_work.add("consciousness")
print(our_work)
{'mindwandering', 'perception', 'metacognition', 'consciousness'}
our_work.add("consciousness")
print(our_work)
our_work.add("consciousness")
print(our_work)
{'mindwandering', 'perception', 'metacognition', 'consciousness'}
{'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)
{'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
Working with strings#
We have already seen strings. Here is another example.
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
my_string.strip()
'a string'
Adding strings:
example + " or two"
'mary had a little lamb or two'
Putting strings into other strings:
subject_id = "sub1"
print(f"Subject {subject_id} is excellent")
Subject sub1 is excellent
age = 29
print(f"Subject {subject_id} is {age} years old")
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.
subject_no = 4
print(f"Subject {subject_no:02d} is here")
Subject 04 is here
For loop#
for i in range(10):
print(i)
0
1
2
3
4
5
6
7
8
9
Indentation is crucial!
for i in range(10):
print(i)
Cell In[46], line 2
print(i)
^
IndentationError: expected an indented block after 'for' statement on line 1
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
strange_election and satisfying_experience
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#
Use pathlib to write text and read text from files.
Write lines to a text file:
from pathlib import Path
path = Path("important_notes.txt")
type(path)
pathlib.PosixPath
my_text = """captains log 672828: I had a banana for breakfast.
captains log 672829: I should watch less TV.
"""
print(my_text)
captains log 672828: I had a banana for breakfast.
captains log 672829: I should watch less TV.
path.write_text(my_text)
96
Read lines from a text file:
text_again = path.read_text()
print(text_again)
captains log 672828: I had a banana for breakfast.
captains log 672829: I should watch less TV.
Split the lines from a text into a list, where there is one element per line, and each element is a string:
# The splitlines method of a string.
lines = text_again.splitlines()
len(lines)
print(lines[0])
print(lines[1])
captains log 672828: I had a banana for breakfast.
captains log 672829: I should watch less TV.
We may want to delete the file when we’ve finished with it. Again the Path
object does the job:
# Delete the file.
path.unlink()