# Division in Python 2.7. and 3.3 [duplicate]

How can I divide two numbers in Python 2.7 and get the result with decimals?

I don't get it why there is difference:

in Python 3:

``````>>> 20/15
1.3333333333333333
``````

in Python 2:

``````>>> 20/15
1
``````

Isn't this a modulo actually?

96

In python 2.7, the `/` operator is integer division if inputs are integers.

If you want float division (which is something I always prefer), just use this special import:

``````from __future__ import division
``````

See it here:

``````>>> 7 / 2
3
>>> from __future__ import division
>>> 7 / 2
3.5
>>>
``````

Integer division is achieved by using `//`, and modulo by using `%`

``````>>> 7 % 2
1
>>> 7 // 2
3
>>>
``````

EDIT

As commented by `user2357112`, this import has to be done before any other normal import.

Tuesday, June 1, 2021

42

Use `dict.setdefault()`:

``````dic.setdefault(key,[]).append(value)
``````

help(dict.setdefault):

``````    setdefault(...)
D.setdefault(k[,d]) -> D.get(k,d), also set D[k]=d if k not in D
``````
Wednesday, June 9, 2021

12

`/` is a different operator in Python 3; in Python 2 `/` alters behaviour when applied to 2 integer operands and returns the result of a floor-division instead:

``````>>> 3/2   # two integer operands
1
>>> 3/2.0 # one operand is not an integer, float division is used
1.5
``````

``````from __future__ import division
``````

to the top of your code to make `/` use float division in Python 2, or use `//` to force Python 3 to use integer division:

``````>>> from __future__ import division
>>> 3/2    # even when using integers, true division is used
1.5
>>> 3//2.0 # explicit floor division
1.0
``````

Using either of these techniques works in Python 2.2 or newer. See PEP 238 for the nitty-gritty details of why this was changed.

Wednesday, July 7, 2021

35

### `yield`

You can use a generator for an elegant solution. At each iteration, yield twice—once with the original element, and once with the element with the added suffix.

The generator will need to be exhausted; that can be done by tacking on a `list` call at the end.

``````def transform(l):
for i, x in enumerate(l, 1):
yield x
yield f'{x}_{i}'  # {}_{}'.format(x, i)
``````

You can also re-write this using the `yield from` syntax for generator delegation:

``````def transform(l):
for i, x in enumerate(l, 1):
yield from (x, f'{x}_{i}') # (x, {}_{}'.format(x, i))
``````

``````out_l = list(transform(l))
print(out_l)
['a', 'a_1', 'b', 'b_2', 'c', 'c_3']
``````

If you're on versions older than python-3.6, replace `f'{x}_{i}'` with `'{}_{}'.format(x, i)`.

Generalising
Consider a general scenario where you have N lists of the form:

``````l1 = [v11, v12, ...]
l2 = [v21, v22, ...]
l3 = [v31, v32, ...]
...
``````

Which you would like to interleave. These lists are not necessarily derived from each other.

To handle interleaving operations with these N lists, you'll need to iterate over pairs:

``````def transformN(*args):
for vals in zip(*args):
yield from vals

out_l = transformN(l1, l2, l3, ...)
``````

### Sliced `list.__setitem__`

I'd recommend this from the perspective of performance. First allocate space for an empty list, and then assign list items to their appropriate positions using sliced list assignment. `l` goes into even indexes, and `l'` (`l` modified) goes into odd indexes.

``````out_l = [None] * (len(l) * 2)
out_l[::2] = l
out_l[1::2] = [f'{x}_{i}' for i, x in enumerate(l, 1)]  # [{}_{}'.format(x, i) ...]
``````

``````print(out_l)
['a', 'a_1', 'b', 'b_2', 'c', 'c_3']
``````

This is consistently the fastest from my timings (below).

Generalising
To handle N lists, iteratively assign to slices.

``````list_of_lists = [l1, l2, ...]

out_l = [None] * len(list_of_lists[0]) * len(list_of_lists)
for i, l in enumerate(list_of_lists):
out_l[i::2] = l
``````

### `zip` + `chain.from_iterable`

A functional approach, similar to @chrisz' solution. Construct pairs using `zip` and then flatten it using `itertools.chain`.

``````from itertools import chain
# [{}_{}'.format(x, i) ...]
out_l = list(chain.from_iterable(zip(l, [f'{x}_{i}' for i, x in enumerate(l, 1)])))
``````

``````print(out_l)
['a', 'a_1', 'b', 'b_2', 'c', 'c_3']
``````

`iterools.chain` is widely regarded as the pythonic list flattening approach.

Generalising
This is the simplest solution to generalise, and I suspect the most efficient for multiple lists when N is large.

``````list_of_lists = [l1, l2, ...]
out_l = list(chain.from_iterable(zip(*list_of_lists)))
``````

### Performance

Let's take a look at some perf-tests for the simple case of two lists (one list with its suffix). General cases will not be tested since the results widely vary with by data.

Benchmarking code, for reference.

### Functions

``````def cs1(l):
def _cs1(l):
for i, x in enumerate(l, 1):
yield x
yield f'{x}_{i}'

return list(_cs1(l))

def cs2(l):
out_l = [None] * (len(l) * 2)
out_l[::2] = l
out_l[1::2] = [f'{x}_{i}' for i, x in enumerate(l, 1)]

return out_l

def cs3(l):
return list(chain.from_iterable(
zip(l, [f'{x}_{i}' for i, x in enumerate(l, 1)])))

def ajax(l):
return [
i for b in [[a, '{}_{}'.format(a, i)]
for i, a in enumerate(l, start=1)]
for i in b
]

def ajax_cs0(l):
# suggested improvement to ajax solution
return [j for i, a in enumerate(l, 1) for j in [a, '{}_{}'.format(a, i)]]

def chrisz(l):
return [
val
for pair in zip(l, [f'{k}_{j+1}' for j, k in enumerate(l)])
for val in pair
]
``````
Thursday, July 15, 2021

89

This question has been addressed on SO, for example, here: Nonalphanumeric list order from os.listdir() in Python

Looks like Python returns the order that the native filesystem uses, and you have to sort them afterwards.

Thursday, August 5, 2021