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Everything will flow: Python control-flow constructs#

Control flow (-statements) controls the order of execution of statements within a program. Python offers a common set of 'control-flow' statements.

choices - the if statement#

The if statement provides condtional-execution of code-blocks, see also Python docs if statement.

if statement example:

>>> a = 1
>>> if a == 0:
...     print('a is 0')
... elif a == 1:
...     print('a is 1')
... elif a == 2:
...     print('a is 2')
... else:
...     print('a neither 0, nor 1 or 2')
... 
>>> a is 1

loops#

for statement#

The for statement is a count-based loop-control, i.e. the number of repetitions is based upon the elements of an "iterable".

An iterable can be e.g. a

  • list,
  • tuple,
  • str,
  • range (e.g. range(0,3) or range(2) or range(1,3)
  • ...

range() for loop examples#

Note: The range() built-in function creates object of type range, which produces a sequence of integers, see Range or help(range):

>>> help(range)
Help on class range in module builtins:

class range(object)
 |  range(stop) -> range object
 |  range(start, stop[, step]) -> range object

... <abbreviated>

range() with start and stop#

>>> for elem in range(2, 6):
...     print(elem)
...
2
3
4
5
>>>

range() with stop#

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

range() with start, stop and step#

>>> for elem in range(0,9,3):
...     print (elem)
... 
0
3
6
>>>

Looping over objects#

Example: for loop using a list object:

>>> for elem in [0, 1, 2]:
...     print(elem)
... 
0
1
2
>>>

You are able to modify a looped-over mutable object:

>>> for elem in my_list:
...     print(elem)
...     del my_list[0]
... 
1
3
>>>

But: Don't do that! You'll create hard to understand code and unexpected behaviour.

while statement#

The while statement is a condition-based loop-control, i.e the number of repetitions is controlled by a boolean-expression evaluating to True.

In a while loop a 'condition variable' is evaluated before running the loop body (and usually changed within the body).

simple while statement#

>>> a = 0
>>> while a < 3:
...     print(a)
...     a += 1   # change the condition variable
... 
0
1
2
>>

break statement#

With the break statement loops can be terminated prematurely.

Usage: Stop iteration at occurence of a condition.

Using a break statement in a for loop#

>>> s = "text"
>>> for elem in s:
...     if elem == 'x':
...         break
...     print(elem)
... 
t
e
>>>

Using a break statement in a while loop#

>>> s = "text"
>>> idx = 0
>>> while idx < len(s):
...     if s[idx] == 'x':
...         break
...     print(s[idx])
...     idx += 1
... 
t
e
>>>

continue statement#

With a continue statement loops can skip the rest of the current iteration.

Usage: 'skip-on-condition'

Using a continue statement in a for loop#

>>> for elem in s:
...     if elem == 'x':
...         continue
...     print(elem)
... 
t
e
t
>>>

Using a continue statement in a while loop#

>> idx = 0
>>> while idx < len(s):
...     if s[idx] == 'x':
...         idx += 1
...         continue
...     print(s[idx])
...     idx += 1
... 
t
e
t
>>>

else clause of loops#

In Python loops can have an optional else clause.

See the Python docs for the purpose of aloop else clause) and when it is executed. Quote:

"Loop statements may have an else clause; it is executed when the loop terminates through exhaustion of the iterable (with for) or when the condition becomes false (with while), but not when the loop is terminated by a break statement"

So the else clause can be seen as 'finalizer' block of statements which are processed ath the end of a loop during normal operations.

The exact workings of the else clause is a bit hard to remember.1 Still, sometimes it comes in handy.

Using else clause in a for loop#

>>> s = "text"
>>> for elem in s:
...     print(elem)
... else:
...     print("End of normal processing")
... 
t
e
x
t
End of normal processing
>>>

whereas:

>>> s = "text"
>>> for elem in s:
...     print(elem)
...     if elem == "x":
...         break
... else:
...     print("End of normal processing")
...
t
e
x
>>>

Using else clause in a while loop#

>>> s = "text"
>>> idx = 0
>>> while idx < len(s):
...     print(s[idx])
...     idx += 1
... else:
...     print("End of normal processing")
... 
t
e
x
t
End of normal processing
>>>

whereas:

>>> s = "text"
>>> idx = 0
>>> while idx < len(s):
...     print(s[idx])
...     if s[idx] == "x":
...         break
...     idx += 1
... else:
...     print("End of normal processing")
...
t
e
>>>

pass statement#

The pass- statement is a noop statement, meaning no-operation, as it does nothing.

Use it where you need a statement syntactically but there is no needed/sensible program action, see the Python Docs on pass statement.

Example - pass statement in a class definition:

>>> class A: pass
... 
>>> type(A)
<class 'type'>
>>>

match statement#

The match statement is a relatively young addition at the time of writing, and we won't cover it in any depth here.

Dubbed 'structural pattern matching' it's new in Python version 3.10 and allows you to match a value ('subject') against one ore more patterns.

When the match subject matches a pattern (a 'match success' or 'pattern success') then the matched values may be bound to names.

In it's simplest form it looks s.th. like this:

>>> def match_traffic_light_status(status):
...     match status:
...         case 'green':
...             print('Go!')
...         case 'red':
...             print('Stop!')
...         case _:
...             print(f'traffic light {status}?!')
... 
>>> match_traffic_light_status('red')
Stop!
>>> match_traffic_light_status('blue')
traffic light blue?!
>>>

In this basic form it bears some similarity to switch-case constructs found in e.g. the C language, but it is way more powerful.

See the Python tutorial and Reference for in-depth knowledge on match.

  1. For one of the authors, at least. 😉 ↩