Things to pay attention to when developing Python

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

1. List comprehension

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

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

For most beginners, Based on previous language experience, you will probably do it like this

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

But there is a better way:

bag = [elem * 2 for elem in bag]

Very concise, right? This is called a Python list comprehension.

2. Traverse the list

Continue with the list above.

If possible avoid doing this:

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

Instead it should be like this:

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 each element, but if you must, use the enumerate function. It looks like this:

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

Very intuitive and clear.

3. Element exchange

If you are switching from Java or C language to Python, you may be used to this:

a = 5 
b = 10

# 交换 a 和 b
tmp = a 
a = b 
b = tmp

But Python provides a more natural and better method!

a = 5 
b = 10 
# 交换a 和 b
a, b = b, a

Pretty enough, right?

4. Initialization list

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

bag = [] 
for _ in range(10): 
 bag.append(0)

Let’s try another way:

bag = [0] * 10

Look, how elegant.

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

For example:

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

Oops! All lists are changed, and we only want to change the first list.

Change it:

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 evil"
Ask yourself, is initializing a list necessary?

5. Constructing strings

You will often need to print strings. If you have a lot of variables, avoid the following:

name = "Raymond" 
age = 22 
born_in = "Oakland, CA" 
string = "Hello my name is " + name + "and I'm " + str(age) + " years old. I was born in " + born_in + "." 
print(string)

Um, how messy does this look? You can use a nice and concise method instead, .format .

Do this:

name = "Raymond" 
age = 22 
born_in = "Oakland, CA" 
string = "Hello my name is {0} and I'm {1} years old. I was born in {2}.".format(name, age, born_in) 
print(string)

Much better!

6. Returning tuples

Python allows you to return multiple elements in a function, which makes life easier. But when unpacking tuples, there is a common error like this:

def binary(): 
 return 0, 1

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

This is not necessary, you can completely change it to this:

def binary(): 
 return 0, 1

zero, one = binary()

If you need all elements to be returned, use an underscore_:

zero, _ = binary()

That’s it So efficient!

7. Access Dicts (dictionaries)

You will also often write keys and pairs (keys, values) into dicts.

If you try to access a key that does not exist in the dict, you may be tempted to do this to avoid KeyError errors:

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(10): 
 if i in countr:
  print("Count of {}: {}".format(i, countr[i]))
 else:
  print("Count of {}: {}".format(i, 0))

However, using get() is a better way.

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(10): 
 print("Count of {}: {}".format(i, countr.get(i, 0)))

Of course you can also use setdefault instead.

This is a simpler but more expensive method:

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(10): 
 print("Count of {}: {}".format(i, countr.get(i, 0)))

You can also use dict derivation.

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

These two methods are expensive because they traverse the list every time count is called.

8 Using libraries

Just import existing libraries and you can do what you really want.

Still talking about the previous example, we build a function to count the number of times a number appears in the list. Well, there is 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(10): 
 print("Count of {}: {}".format(i, countr[i]))

Some reasons for using the library:

1. The code is correct and tested.

2. Their algorithm may be optimal, so it can run faster.

3. Abstraction: They are clearly pointed and documented, and 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 the start point and stop point, 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 slicing, we specify the stop point is 5, and then stop Will remove 5 elements from the list.

What to do if it is the last 5 elements?

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

Don’t you understand? -5 means take 5 elements from the end of the list.

If you want to operate on the intervals between elements 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 it this way:

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

# 或者用 ranges
bag = list(range(0,10,2)) 
print(bag)

That’s it for the steps in the list. list[::2] means traversing the list and taking out an element in two steps.

You can use list[::-1] to do a cool flipping list.

10. Tab key or space key

In the long run, mixing tabs and spaces will cause disaster, and you will see IndentationError: unexpected indent. Whether you choose the tab key or the space bar, you should keep using it throughout your files and projects.

One reason to use spaces instead of tabs is that tabs are not the same in all editors. Depending on the editor used, tabs may be treated as 2 to 8 spaces.

You can also use spaces to define tabs when writing code. This way you can choose how many spaces to use as tabs. Most Python users use 4 spaces.

Summary

The above are the tips that you should pay attention to in Python development. I hope it will be helpful to everyone in learning and using python. If you have any questions, you can leave a message to communicate.

For more related articles that you should pay attention to in Python development, please pay attention to the PHP Chinese website!