Python: Introduction¶

This notebook provides basic information to get you started. You should read the Python tutorial carefully if you want to master Python.

Comments¶

In [1]:

# This is a python comment

a = 3 # Another comment

Basic types¶

Numbers¶

In [2]:

1234567891011121314151617181920

Out[2]:

1234567891011121314151617181920

In [3]:

3.14

Out[3]:

3.14

In [4]:

2 + 3j

Out[4]:

(2+3j)

Booleans¶

In [5]:

True

Out[5]:

True

In [6]:

False

Out[6]:

False

Strings¶

In [7]:

'hello, world'

Out[7]:

'hello, world'

In [8]:

"hello, world"

Out[8]:

'hello, world'

In [9]:

"""hello,
world"""

Out[9]:

'hello,\nworld'

In [10]:

'''hello,
world'''

Out[10]:

'hello,\nworld'

In [11]:

b'hello world'

Out[11]:

b'hello world'

In [12]:

name = 'world'
f'hello {name}'

Out[12]:

'hello world'

None¶

In [13]:

None

Basic collection¶

Lists¶

In [14]:

[1,2,3]

Out[14]:

[1, 2, 3]

Tuples¶

In [15]:

(1,2,3)

Out[15]:

(1, 2, 3)

Dictionaries¶

In [16]:

{'a': 1, 'b': 2, 'c': 3}

Out[16]:

{'a': 1, 'b': 2, 'c': 3}

Operators¶

Arithmetic operators¶

In [17]:

1 + 2

Out[17]:

In [18]:

1 - 2

Out[18]:

-1

In [19]:

1 * 2

Out[19]:

In [20]:

1 / 2

Out[20]:

0.5

In [21]:

1 // 2

Out[21]:

In [22]:

7 % 3

Out[22]:

In [23]:

7**3

Out[23]:

Relational operators¶

In [24]:

2 < 2, 2 > 2

Out[24]:

(False, False)

In [25]:

2 <= 2

Out[25]:

True

In [26]:

2 == 2

Out[26]:

True

In [27]:

2 != 2

Out[27]:

False

Logical operators¶

In [28]:

not True

Out[28]:

False

In [29]:

True and False

Out[29]:

False

In [30]:

True or False

Out[30]:

True

Bitwise¶

In [31]:

16 << 2

Out[31]:

In [32]:

16 >> 2

Out[32]:

In [33]:

16 | 4

Out[33]:

In [34]:

bin(16 | 4)

Out[34]:

'0b10100'

In [35]:

bin(16 & 4)

Out[35]:

'0b0'

In [36]:

bin(16 ^ 4)

Out[36]:

'0b10100'

In [37]:

bin(16)

Out[37]:

'0b10000'

Assignment and augmented assignment¶

In [38]:

a = 3

In [39]:

b = a

In [40]:

a, b

Out[40]:

(3, 3)

In [41]:

a = a + 3

In [42]:

a, b

Out[42]:

(6, 3)

In [43]:

a = [1,2,3]

In [44]:

b = a

In [45]:

a, b

Out[45]:

([1, 2, 3], [1, 2, 3])

In [46]:

a[1] = 12

In [47]:

a, b

Out[47]:

([1, 12, 3], [1, 12, 3])

In [48]:

c = 1
c += 2

In [49]:

Out[49]:

In [50]:

c *= 3

In [51]:

Out[51]:

Identity and membership¶

In [52]:

a = 3
b = 3

In [53]:

a is b

Out[53]:

True

In [54]:

a == b

Out[54]:

True

In [55]:

id(a), id(b)

Out[55]:

(4409367312, 4409367312)

In [56]:

a = [1,2,3]
b = [1,2,3]

In [57]:

a is b

Out[57]:

False

In [58]:

a == b

Out[58]:

True

In [59]:

id(a), id(b)

Out[59]:

(4449129352, 4449130760)

In [60]:

2 in [1,2,3]

Out[60]:

True

In [61]:

2 not in [1,2,3]

Out[61]:

False

In [62]:

2 in {'a': 1, 'b': 2}

Out[62]:

False

In [63]:

2 in {1: 'a', 2: 'b'}

Out[63]:

True

Naming conventions¶

In [64]:

NAMES_FOR_CONSTANTS = None

In [65]:

variable_and_function_names = None

In [66]:

ClassNamesAreInCamelCase = None

Control Flow¶

if-elif-else¶

In [67]:

day = 'sunny'

if day == 'rainy':
    print('Bring an umbrella')

In [68]:

if day == 'rainy':
    print('Bring an umbrella')
else:
    print('Come as you are')

Come as you are

In [69]:

if day == 'rainy':
    print('Bring an umbrella')
elif day == 'sunny':
    print('Bring sunglasses')
elif day == 'snowing':
    print('Stay home')
else:
    print('Come as you are')

Bring sunglasses

The ternary operator¶

In [70]:

condition = 2 + 2 == 4
True if condition else False

Out[70]:

True

for¶

In [71]:

for x in [1,2,3]:
    print(x**2)

1
4
9

while¶

In [72]:

n = 3
while n > 0:
    print(n)
    n -= 1

3
2
1

break¶

In [73]:

n = 10
while n > 0:
    print(n)
    n -= 1
    if n == 7:
        break

10
9
8

continue¶

In [74]:

for i in [1,2,3,4,5]:
    if i % 2 == 0:
        continue
    print(i**2)

1
9
25

Input and output¶

Reading text files¶

In [75]:

%%file test_file.txt
This old man
He played one
He played knick-knack on my thumb

Writing test_file.txt

The standard way is to treat the file handler as an iterator of lines¶

In [76]:

with open('test_file.txt', 'r') as f:
    for line in f:
        print(line, end='')

This old man
He played one
He played knick-knack on my thumb

For small files, you may choose to read into memory as a single string¶

In [77]:

with open('test_file.txt', 'r') as f:
    text = f.read()
text

Out[77]:

'This old man\nHe played one\nHe played knick-knack on my thumb'

Or as a list of lines¶

In [78]:

with open('test_file.txt', 'r') as f:
    text = f.readlines()
text

Out[78]:

['This old man\n', 'He played one\n', 'He played knick-knack on my thumb']

Appending to text files¶

In [79]:

with open('test_file.txt', 'a') as f:
    f.write('\nThis old man\n')
    f.write('He played two\n')
    f.write('He played knick-knack on my shoe\n')

In [80]:

! cat test_file.txt

This old man
He played one
He played knick-knack on my thumb
This old man
He played two
He played knick-knack on my shoe

Writing to text files¶

In [81]:

with open('test_file.txt', 'w') as f:
    f.write('This old man\n')
    f.write('He played three\n')
    f.write('He played knick-knack on my knee\n')

In [82]:

! cat test_file.txt

This old man
He played three
He played knick-knack on my knee

Errors and exceptions¶

Examples of common errors and exceptions¶

In [83]:

prnt(123)

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-83-40e9627afc71> in <module>()
----> 1 prnt(123)

NameError: name 'prnt' is not defined

In [84]:

for (i in 1:5) {
    print(i)
}

  File "<ipython-input-84-137ebde5c214>", line 1
    for (i in 1:5) {
               ^
SyntaxError: invalid syntax

In [85]:

for i in [1,23]:
print(i)

  File "<ipython-input-85-831c2af95670>", line 2
    print(i)
        ^
IndentationError: expected an indented block

In [86]:

x = 1
if x ==1:
    print(2)
    else:
        print(3)

  File "<ipython-input-86-8f7c872dc949>", line 4
    else:
       ^
SyntaxError: invalid syntax

In [87]:

xs = [1,2,3]
xs[3]

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-87-cd06bc19b999> in <module>()
      1 xs = [1,2,3]
----> 2 xs[3]

IndexError: list index out of range

In [88]:

d = {'a': 1, 'b': 2}
d['c']

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-88-c9ac53e894ce> in <module>()
      1 d = {'a': 1, 'b': 2}
----> 2 d['c']

KeyError: 'c'

In [89]:

1/0

---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
<ipython-input-89-05c9758a9c21> in <module>()
----> 1 1/0

ZeroDivisionError: division by zero

In [90]:

with open('foobar.txt', 'r'):
    for line in f:
        print(line)

---------------------------------------------------------------------------
FileNotFoundError                         Traceback (most recent call last)
<ipython-input-90-8fccf0e2e539> in <module>()
----> 1 with open('foobar.txt', 'r'):
      2     for line in f:
      3         print(line)

FileNotFoundError: [Errno 2] No such file or directory: 'foobar.txt'

try-except-finally¶

In [91]:

n = 0
try:
    x = 1/n
except ZeroDivisionError as e:
    print(e)

division by zero

Can catch different types of exceptions¶

In [92]:

try:
    xs = [1,2,3]
    xs[3]
except ZeroDivisionError as e:
    print(e)
except IndexError as e:
    print(e)
except KeyError as e:
    print(e)

list index out of range

The `finally` clause is normally used for resource clean-up¶

Code in finally block will be executed regardless of whetehr an exception occurred.

In [93]:

try:
    f = open('foobar.txt', 'w')
    f.write(str(1/0))
except Exception as e:
    print(e)
finally:
    f.close()

division by zero

Clean-up¶

In [94]:

! rm foobar.txt test_file.txt