Python code examples

I’ve met many weird examples of  behaviour in python language while working on Open Event project. Today I’d like to share some examples with you. I think this knowledge is necessary, if you’d like to increase a  bit your knowledge in python area.

Simple adding one element to python list:

def foo(value, x=[]):
  x.append(value)
  return x

>>> print(foo(1))
>>> print(foo(2))
>>> print(foo(3, []))
>>> print(foo(4))

OUTPUT

[1] 
[1, 2] 
[3]
[1, 2, 4]

First output is obvious, but second not exactly. Let me explain it, It happens because x(empty list) argument is only evaluated once, So on every call foo(), we modify that list, appending a value to it. Finally we have [1,2, 4] output. I recommend to avoid mutable params as default.

Another example:

Do you know which type it is?

>>> print(type([ el for el in range(10)]))
>>> print(type({ el for el in range(10)}))
>>> print(type(( el for el in range(10))))

Again first and second type are obvious <class ‘list’>, <class ‘set’>. You may  think that last one should return type tuple but it returns a generator <class ‘generator’>.

Example:

Do you think that below code returns an exception?

list= [1,2,3,4,5]
>>> print(list [8:])

If you think that above expression returns index error you’re wrong. It returns empty list [].

Example funny boolean operators

>>> 'c' == ('c' or 'b')
True
>>> 'd' == ('a' or 'd')
False
>>> 'c' == ('c' and 'b')
False 
>>> 'd' == ('a' and 'd')
True

You can think that that OR and AND operators are broken.

You have to know how python interpreter behaves while looking for OR and AND operators.

So OR Expression takes the first statement and checks if it is true. If the first statement is true, then Python returns object’s value without checking second value. If first statement is false interpreter checks second value and returns that value.

AND operator checks if first statement is false, the whole statement has to be false. So it returns first value, but if first statement is true it checks second statement and returns second value.

Below i will show you how it works

>>> 'c' == ('c' or 'b')
>>> 'c' == 'c'
True
>>> 'd' == ('a' or 'd')
>>> 'd' == 'a'
False
>>> 'c' == ('c' and 'b')
>>> 'c' == 'b'
False 
>>> 'd' == ('a' and 'd')
>>> 'd' == 'd'
True

I hope that i have explained you how the python interpreter checks OR and AND operators. So know above examples should be more understandable.