[PYTHON] Learning record (4th day) #How to get the absolute path from the relative path

content of study

• How to get an absolute path from a relative path
• Python basic grammar

How to get an absolute path from a relative path

When the link destination is described as a relative path in HTML, use ʻurllib.parse.urljon () `to get the absolute path.

Description example

from urllib.parse import urljoin

base = "http://exsample.com/html/a.html"

compurl = lambda q: print(urljoin(base,q))
compurl("b.html")
compurl("sub/c.html")
compurl("../index.html")
compurl("../img/hoge.png ")

Execution result

http://example.com/html/b.html http://example.com/html/sub/c.html http://example.com/index.html http://example.com/img/hoge.png

Python basic grammar

pass statement

In Python, processing blocks are defined by indentation, so when there is no need to perform processing, the processing block itself disappears. Therefore, the pass statement is used to explicitly describe that nothing is to be done.

with open('exsample.txt', 'w'):
    pass

Conditional branch

Describe using ʻif, ʻelif, ʻelse`. In addition, when branching into two processing blocks by one conditional expression, it can be described using a ternary operator.

 if conditional expression 1:
 <Processing block 1>
 #Processing executed when conditional expression 1 is True
 elif conditional expression 2:
 <Processing block 2>
 #Processing executed when conditional expression 1 is False and conditional expression 2 is True
else:
 <Processing block 3>
 #Processing executed when conditional expression 1 is False, conditional expression 2 is False, and conditional expression 3 is True.

# Conditional branching by ternary operator
# Value 1 when the conditional expression is True, value 2 when False
 Value 1 if conditional expression else value 2

Iterator

Hold the data as a rule that can generate a series of data, not the value itself that represents the data. When a large amount of data is required, iterators are more memory efficient. A description example of a range type iterator is shown below.

# The following three range () return iterators increasing by 1 from 0 to 9.
range(0, 10, 1)
range(0, 10)
a = range(10)

print(a)
print(a[0])

Execution result

range(0, 10) 0

range () has the function of the increment operator (++) which cannot be used in Python. Decrement (--) is done using an iterator called reversed ().

break and continue statements

By writing in the processing block of the loop statement, the running loop statement can be controlled.

break statement

Break the currently running loop and exit the loop.

b = 0
while True:
    b += 1
    if b > 5
 When break # b = 6, the loop is exited and the process ends.
    print(b)

Execution result

1 2 3 4 5

continue statement

Suspends the processing block being executed and moves the processing to the conditional expression evaluation of the loop.

c = 0
while True:
    c += 1
    if c < 5
 The subsequent processing will not be executed until continue # b = 6.
    print(c)
    break

Execution result

6

Summary

I started having a hard time understanding the code in the scraping reference book I was using, so I decided to start learning the basic grammar of Python again. I was surprised that increment decrement, which was very convenient in C and Java, cannot be used, but it can be handled well by using cumulative assignment (+ =, -=) and iterators. I want to do it.

Reference book

I have attached the GitHub published from the book I referred to. Supplementary revision Python scraping & machine learning development technique