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

>>> 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:

>>> a = 1

>>> b = 1.

>>> a == b

True

>>> a is b

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

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?

Warning

Not safe to modify the sequence you are iterating over.

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')

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 is random, thus we use sorted() which will sort on the keys.

1.2.3.6. List Comprehensions

Instead of creating a list by means of a loop, one can make use of a list comprehension with a rather self-explaining syntax.

>>> [i**2 for i in range(4)]

[0, 1, 4, 9]