1.2.3. Control Flow

Controls the order in which the code is executed.

1.2.3.1. if/elif/else

>>> if 2**2 == 4:
... print('Obvious!')
...
Obvious!

Blocks are delimited by indentation

Type the following lines in your Python interpreter, and be careful to respect the indentation depth. The Ipython shell automatically increases the indentation depth after a colon : sign; to decrease the indentation depth, go four spaces to the left with the Backspace key. Press the Enter key twice to leave the logical block.

>>> a = 10
>>> if a == 1:
... print(1)
... elif a == 2:
... print(2)
... else:
... print('A lot')
A lot

Indentation is compulsory in scripts as well. As an exercise, re-type the previous lines with the same indentation in a script condition.py, and execute the script with run condition.py in Ipython.

1.2.3.2. for/range

Iterating with an index:

>>> for i in range(4):
... print(i)
0
1
2
3

But most often, it is more readable to iterate over values:

>>> for word in ('cool', 'powerful', 'readable'):
... print('Python is %s' % word)
Python is cool
Python is powerful
Python is readable

1.2.3.3. while/break/continue

Typical C-style while loop (Mandelbrot problem):

>>> z = 1 + 1j
>>> while abs(z) < 100:
... z = z**2 + 1
>>> z
(-134+352j)

More advanced features

break out of enclosing for/while loop:

>>> z = 1 + 1j
>>> while abs(z) < 100:
... if z.imag == 0:
... break
... z = z**2 + 1

continue the next iteration of a loop. :

>>> a = [1, 0, 2, 4]
>>> for element in a:
... if element == 0:
... continue
... print(1. / element)
1.0
0.5
0.25

1.2.3.4. Conditional Expressions

if <OBJECT>:
Evaluates to False:
  • any number equal to zero (0, 0.0, 0+0j)
  • an empty container (list, tuple, set, dictionary, ...)
  • False, None
Evaluates to True:
  • everything else
a == b:

Tests equality, with logics:

>>> 1 == 1.
True
a is b:

Tests identity: both sides are the same object:

>>> 1 is 1.
False
>>> a = 1
>>> b = 1
>>> a is b
True
a in b:

For any collection b: b contains a

>>> b = [1, 2, 3]
>>> 2 in b
True
>>> 5 in b
False

If b is a dictionary, this tests that a is a key of b.

1.2.3.5. Advanced iteration

1.2.3.5.1. Iterate over any sequence

You can iterate over any sequence (string, list, keys in a dictionary, lines in a file, ...):

>>> vowels = 'aeiouy'
>>> for i in 'powerful':
... if i in vowels:
... print(i)
o
e
u
>>> message = "Hello how are you?"
>>> message.split() # returns a list
['Hello', 'how', 'are', 'you?']
>>> for word in message.split():
... print(word)
...
Hello
how
are
you?

Few languages (in particular, languages for scientific computing) allow to loop over anything but integers/indices. With Python it is possible to loop exactly over the objects of interest without bothering with indices you often don’t care about. This feature can often be used to make code more readable.

Warning

Not safe to modify the sequence you are iterating over.

1.2.3.5.2. Keeping track of enumeration number

Common task is to iterate over a sequence while keeping track of the item number.

  • Could use while loop with a counter as above. Or a for loop:

    >>> words = ('cool', 'powerful', 'readable')
    
    >>> for i in range(0, len(words)):
    ... print((i, words[i]))
    (0, 'cool')
    (1, 'powerful')
    (2, 'readable')
  • But, Python provides a built-in function - enumerate - for this:

    >>> for index, item in enumerate(words):
    
    ... print((index, item))
    (0, 'cool')
    (1, 'powerful')
    (2, 'readable')

1.2.3.5.3. Looping over a dictionary

Use items:

>>> d = {'a': 1, 'b':1.2, 'c':1j}
>>> for key, val in sorted(d.items()):
... print('Key: %s has value: %s' % (key, val))
Key: a has value: 1
Key: b has value: 1.2
Key: c has value: 1j

Note

The ordering of a dictionary in random, thus we use sorted() which will sort on the keys.

1.2.3.6. List Comprehensions

>>> [i**2 for i in range(4)]
[0, 1, 4, 9]

Exercise

Compute the decimals of Pi using the Wallis formula:

\pi = 2 \prod_{i=1}^{\infty} \frac{4i^2}{4i^2 - 1}