Share 10 tips to note in Python development _python

Please note: This article assumes that we are all using Python 3

1. List derivation type

You have a list:bag = [1, 2, 3, 4, 5]

Now you want to double all the elements to make it look like this: [2, 4, 6, 8, 10]

Most beginners, according to previous language experience will probably do this

Bag = [1, 2, 3, 4, 5] for 
i in range (len (bag)): 
 bag[i] = bag[i] * 2

But there's a better way:

Bag = [Elem * 2 for Elem in bag]

It's neat, isn't it? This is called a list derivation of Python.

2. Traverse List

Continue, or the list above.

If possible avoid this:

Bag = [1, 2, 3, 4, 5] for 
i in range (len (bag)): 
 print (Bag[i])

Instead, it should be:

Bag = [1, 2, 3, 4, 5] for 
i in bag: 
 print (i)

If x is a list, you can iterate over its elements. In most cases you don't need the index of the elements, but if you have to do this, use a enumerate function. It looks like the bottom:

Bag = [1, 2, 3, 4, 5] 
for index, element in enumerate (bag): 
 print (index, Element)

Very intuitive and clear.

3. Element Interchange

If you go from Java or C to Python, you might get used to it:

A = 5 
B =

exchange A and b
tmp = a 
a = b 
b = tmp

But Python offers a more natural and better way!

A = 5 
B = Ten 
exchange A and B
A, B = B, a

Beautiful, isn't it?

4. Initializing the list

If you want a list of 10 integers 0, you might first think:

Bag = [] for 
_ in range (a): 
 bag.append (0)

Put it another way:

Bag = [0] * 10

Look, how elegant.

Note: If your list contains a list, doing so will produce a shallow copy.

As an example:

Bag_of_bags = [[0]] * 5 # [[0], [0], [0], [0], [0]] 
bag_of_bags[0][0] = 1 # [[1], [1], [1], [1], [1]]

oops! All the listings are changed, and we just want to change the first list.

Change a change:

Bag_of_bags = [[0] for _ in range (5)] 
# [[0], [0], [0], [0], [0]]

bag_of_bags[0][0] = 1 
# [[1], [0], [0], [0], [0]]

Also remember:

"Premature optimization is the root of all evils."
Ask yourself, is it necessary to initialize a list?

5. Construct string

You will often need to print strings. If there are many variables, avoid the following:

Name = "Raymond" Age 
= 
born_in = "Oakland, CA" 
string = "Hello My name is" + name + "and I ' m" + STR (AG E) + "years old." I was born in "+ Born_in +". " 
Print (String)

Well, how messy does it look? You can replace it with a nice, concise way .format .

Do this:

Name = "Raymond" Age 
= 
born_in = "Oakland, CA" 
string = "Hello I name is {0} and I ' m {1} years old. I was born in {2}. ". Format (name, age, born_in) 
print (String)

Much better!

6. Return to tuples (tuple)

Python allows you to return multiple elements in a function, which makes life simpler. But there are common errors that come out of qualifying when you unpack the tuples:

def binary (): return 
 0, 1 result

= binary () 
zero = result[0] one 
= result[1]

This is not necessary, you can completely replace this:

def binary (): return 
 0, 1

zero, one = binary ()

If you need all the elements to be returned, use an underscore _:

Zero, _ = Binary ()

It's so efficient!

7. Visit Dicts (dictionary)

You will also often write to Dicts key , pair (keys, values).

If you try to access a non-existent one dict key , you may be tempted to do so in order to avoid the KeyError error:

Countr = {} 
bag = [2, 3, 1, 2, 5, 6, 7, 9, 2, 7] for 
i in bag: 
 if I in Countr:
  countr[i] = 1
 else:
   countr[i] = 1 for

i in range: 
 if I in Countr:
  print ("Count of {}: {}". Format (i, Countr[i]))
 Else :
  print ("Count of {}: {}". Format (i, 0))

But it get() 's a better way to do it.

Countr = {} 
bag = [2, 3, 1, 2, 5, 6, 7, 9, 2, 7] for 
i in bag: 
 countr[i] = countr.get (i, 0) + 1 for

I In range: 
 print ("Count of {}: {}". Format (i, Countr.get (i, 0))

Of course you can use it setdefault instead.

It also uses a simpler but more costly approach:

Bag = [2, 3, 1, 2, 5, 6, 7, 9, 2, 7] 
countr = dict ([(NUM, Bag.count (num)) for num in bag]) for

i in range: 
   
    print ("Count of {}: {}". Format (i, Countr.get (i, 0))
   

You can also use the dict deduced formula.

Countr = {num:bag.count (num) for num in bag}

Both of these methods are expensive because they traverse the list every time they count are invoked.

8 Using libraries

Existing libraries just import you can do what you really want to do.

Or the previous example, we built a function to count the number of occurrences of a number in the list. Well, there's already a library that can do such a thing.

From collections import Counter 
bag = [2, 3, 1, 2, 5, 6, 7, 9, 2, 7] 
countr = Counter (bag) for

I in range (1 0): 
 print ("Count of {}: {}". Format (i, Countr[i])

Some reasons to use the library:

1, the code is correct and tested.

2, their algorithm may be optimal, so that the faster run.

3, Abstract: They point to clear and document-friendly, you can focus on those that have not yet been implemented.

4, in the end, it's already there, you don't have to reinvent the wheel.

9. Slicing/stepping in the list

You can specify start the points and stop points, just like this list[start:stop:step] .

We take out the first 5 elements in the list:

Bag = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] for 
Elem in Bag[:5]: 
 print (Elem)

This is the slice, we specify the stop point is 5, before stopping will be removed from the list of 5 elements.

What if the last 5 elements were to be done?

Bag = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] for 
Elem in bag[-5:]: 
 print (Elem)

Didn't see it, understood? -5 means to remove 5 elements from the end of the list.

If you want to operate on the element interval in the list, you might do this:

Bag = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] for 
index, elem in Enumerate (bag): 
 If index% 2 = 0:
  print (Elem)

But you should do this:

Bag = [0, 2, 3, 4, 5, 6, 7, 8, 9] for 
Elem in bag[::2]: 
 print (Elem)

# or with ranges bag
= List (0,10, 2) 
print (bag)

This is the stepping in the list. list[::2]it means to iterate through the list and take out an element two steps at a time.

You can use list[::-1] a cool flip list.

The TAB key or the SPACEBAR

For a long time, the combination of tab and space can cause disaster, as you will see IndentationError: unexpected indent . Whether you choose the TAB key or the SPACEBAR, you should keep it in your files and projects.

One reason to use a space instead of the TAB key is that the tab is not the same in all editors. Depending on the editor used, tab may be treated as 2 to 8 spaces.

You can also use spaces to define tab in writing code. This way you can choose to use a few spaces as tab. Most Python users use 4 spaces.

Summarize

The above is for you to summarize the Python development to note the tips, I hope to learn and use Python can help, if you have questions can be exchanged message.