Grammer Tips for Python 3

In this article, I would like to share some Python Grammer tips for beginner.

🚕 String

In Python, there are two type of string.

• str = u'...'
  • A sequence of Unicode characters (UTF-16 or UTF-32). It can recognize one character as one character.
• bytes = b'...'
  • A sequence of one byte data (integers between 0 and 255)

# str => byte
'あ'.encode() #=> b'\xe3\x81\x82'
'\uFEFF'.encode('UTF-8') #=> b'\xef\xbb\xbf'

# byte => str
b'\xe3\x81\x82'.decode('UTF-8') #=> 'あ'
b'\xE2\x82\xAC'.decode('UTF-8') # => '€'

Search substring

s = "This be a string"
if s.find("is") == -1:
    print "No 'is' here!"
else:
    print "Found 'is' in the string."

Check whether string starts

str = "this is string example....wow!!!";
print str.startswith( 'this' ) #=> True

Convert string to lowercase

s = "Kilometer"
print(s.lower()) #=> kilometer

Split string to list

x = 'blue,red,green'
x.split(',') #=> ['blue', 'red', 'green']

Remove all line breaks from a long string of text

'test\n\r'.replace('\n', ' ').replace('\r', '') #=> 'test'

Variable expansion in String

number=5
text='test'
'Number is {0}、text is {1}'.format(number, text)

Search a substring

s = "This be a string"
if s.find("is") == -1:
  print "No 'is' here!"

if "is" in s:
  print "No 'is' here!"

Replace substring

This method returns a copy of the string with all occurrences of substring old replaced by new.

str = "this is string example....wow!!! this is really string"
print str.replace("is", "was") #=> thwas was string example....wow!!! thwas was really string
print str.replace("is", "was", 3) #=> thwas was string example....wow!!! thwas is really string

Trim whitespace

' Hello '.strip() #=> 'Hello'
' Hello'.strip() #=> 'Hello'
'Bob has a cat'.strip() #=> 'Bob has a cat'

Embedding variable in Python 3.6 or later

a = 'sample'
print(f'{a}') #=> 'sample'

🗻 List/Array

List comprehension

A list comprehension consists of the following parts:

• An Input Sequence.
• A Variable representing members of the input sequence.
• An Optional Predicate expression.

[i for i in range(10)] #=> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
[i for i in range(10) if i%2==0] #=> [0, 2, 4, 6, 8]
[ i if i%2==0 else str(i) for i in range(10)]  #=> [0, '1', 2, '3', 4, '5', 6, '7', 8, '9']

Append element

lst = ['a']
lst.append('b') #=> ['a', 'b']

Get the number of elements in a list

>> len([1,2,3]) #=> 3

Convert list to string

list1 = ['1', '2', '3']
str1 = ', '.join(list1) #=> '1, 2, 3'

Flatten

Converting multi dimensional array to flatten array is as follows:

l = [[1, 2, 3], [4, 5, 6], [7], [8, 9]]
for sublist in l:
    for item in sublist:
        flat_list.append(item)

How to make a comma-separated string from a list of strings

myList = ['a','b','c','d']
myString = ','.join(myList)

Convert list to set

lst = ['a', 'b', 'b', 'c']
s = set(lst) #=> {'a', 'b', 'c'}

🚌 Dict

Wether find key

if "key" in dict:
  # do something

🐞 Float/Decimal

Convert float to Decimal

format(f, ".15g")

🍄 Tuple

Add variable to tuple

a = (1, 2, 3)
b = a + (4,) #=> (1, 2, 3, 4)

🐯 Regex

Extract pattern matches

r'...' is literal for Regex. It is better to use it for Regex.

import re

m = re.search(r'(?<=-)\w+'< span>, 'spam-egg')m.group(0) #=> 'egg'
m.group(0) #=> 'egg'

p = re.compile('name (.*) is valid')
s = """
someline abc
someother line
name my_user_name is valid
some more lines"""
p.findall(s) #=> ['my_user_name']

string.replace with regular expression

import re

line = re.sub(
           r"(?i)^.*interfaceOpDataFile.*$",
           "interfaceOpDataFile %s" % fileIn,
           line
       )

Remove specific pattern from text

This is example for removing HTML tags/formatting from a string:

import re
def strip_html_tag(data):
    p = re.compile(r'<.*?>')
    return p.sub('', data)

strip_html_tag('I Want This text!')
#=> 'I Want This text!'

🚜 Datetime

Now

import datetime
datetime.datetime.now()

String to Datetime

pip install dateutil and add the following code:

from dateutil import parser
dt = parser.parse("Aug 28 1999 12:00AM")

Convert unix timestamp to readable date

from datetime import datetime

# No miliseconds
datetime.fromtimestamp(1172969203.1)

# With miliseconds
s = 1236472051807 / 1000.0
datetime.fromtimestamp(s).strftime('%Y-%m-%d %H:%M:%S.%f') #=> '2009-03-08 09:27:31.807000'

Get current time in milliseconds

import time
millis = int(round(time.time() * 1000))
print millis

Convert datetime to unix time

datetime.datetime(2012,4,1,0,0).timestamp()

🎃 Number(Float/Int)

convert x to a plain integer

int(x)

🏀 Grammar

Structure for sub package

The following code is little strange:

sys.path.append(os.path.dirname(os.path.abspath(__file__)) + '/../sub_package1')
import utils

Installing the whole root package and placing the sub packages under it will make the loading of the module natural.

mkdir sample_project/root_package
touch sample_project/root_package/__init__.py
mv sample_project/{sub_package1, sub_package2}

The the above directory structure supports the following loading pattern.

from root_package.sub_package1 import SubPackage1

@classmethod

With @classmethod, we don’t have to specify the class name internally.

@classmethod
def create_table(cls, lambda_name):
    Base._create_table(
        cls.__table_name(lambda_name),
        cls.KEY_SCHEMA,
        cls.ATTRIBUTE_DEFINITION
    )

None is instead of null

In Python, the null object is the singleton None.

Ternary operator

x = "OK" if n == 10 else "NG"

Throw/Raise Error

raise RuntimeError('specific message') from error

super().__init__() methods

class Polygon(object):
    def __init__(self, id):
        self.id = id

class Rectangle(Polygon):
    def __init__(self, id, width, height):
        super(self.__class__, self).__init__(id)
        self.shape = (width, height)

Keyword argument for method

def func(a, b, *, option1=1, option2=2):
    pass

func(a, b, option1=10, option2=20)
func(a, b, c) # エラー 固定変数は２つ
func(a, b, opton1=10) # エラー 存在しないキーワード

How to import the class in same dir

from .user import User

Import a module given the full path

import sys
sys.path.append('/foo/bar/mock-0.3.1')
import Mock

Get type information of object

isinstance([0, 10, 20, 30], list) #=> True
isinstance(50, list) #=> False

type([]) is list #=> True
type({}) is dict #=> True

str = "test"
type(str)

Object unique identifier

id(x)

🍮 Etc

Load multiple JSON object

JSON decoder can not load multiple JSON object like this:

{"name": "foo"}
{"name": "bar"}

It makes some error json.decoder.JSONDecodeError: Extra data..

If you want to load multiple JSON object, please use as follows:

import json
from json.decoder import WHITESPACE


def loads_iter(s):
    size = len(s)
    decoder = json.JSONDecoder()

    end = 0
    while True:
        idx = WHITESPACE.match(s[end:]).end()
        i = end + idx
        if i >= size:
            break
        ob, end = decoder.raw_decode(s, i)
        yield ob

Generate CSV file

Generate CSV file from list(array):

import csv

items = [
    ['John', 28],
    ['Sara', 22]
]

with open('/app/job_items.csv', 'w', encoding='utf8') as f:
    wr = csv.writer(f, quoting=csv.QUOTE_ALL, lineterminator='\n')
    wr.writerow(['name', 'age'])
    for i in items:
        wr.writerow(i)

        # Write string data as CSV
        # csvwriter.writerow([i])

Write data to file

file = open(“testfile.txt”, ”w”)  
file.write(“Hello World”)
file.close()

How to access environment variables

import os
print(os.environ['HOME'])

Check existence of an environment variable

if os.environ.get('abc'):
    #abc is set to True

Memoization

from functools import lru_cache

@lru_cache(maxsize=1000)
def fib_memoized(n):
    if n < 2:
        return n
    else:
        return fib(n - 1) + fib(n - 2)

Convert generator object to list

lst = list(gen)

🎳 References

• https://stackoverflow.com/questions/1712227/how-to-get-the-number-of-elements-in-a-list-in-python
• https://stackoverflow.com/questions/5618878/how-to-convert-list-to-string
• https://stackoverflow.com/questions/16566268/remove-all-line-breaks-from-a-long-string-of-text
• https://stackoverflow.com/questions/3437059/does-python-have-a-string-contains-substring-method
• https://tonari-it.com/python-str-format/
• https://qiita.com/junkoda/items/bfd35793c5cd33c600bc
• https://note.nkmk.me/python-type-isinstance/
• https://stackoverflow.com/questions/15340582/python-extract-pattern-matches
• https://stackoverflow.com/questions/2052390/manually-raising-throwing-an-exception-in-python
• https://stackoverflow.com/questions/6269765/what-does-the-b-character-do-in-front-of-a-string-literal
• https://stackoverflow.com/questions/31019854/typeerror-cant-use-a-string-pattern-on-a-bytes-like-object-in-re-findall
• https://stackoverflow.com/questions/1380860/add-variables-to-tuple
• https://stackoverflow.com/questions/3289601/null-object-in-python
• https://qiita.com/howmuch515/items/bf6d21f603d9736fb4a5
• https://stackoverflow.com/questions/12451431/loading-and-parsing-a-json-file-with-multiple-json-objects-in-python
• https://pod.hatenablog.com/entry/2017/08/31/035140
• https://stackoverflow.com/questions/952914/making-a-flat-list-out-of-list-of-lists-in-python
• http://www.pythonforbeginners.com/files/reading-and-writing-files-in-python
• https://stackoverflow.com/questions/17053099/how-to-print-current-date-on-python3
• https://stackoverflow.com/questions/3790848/fastest-way-to-convert-an-iterator-to-a-list
• https://stackoverflow.com/questions/1816880/why-does-csvwriter-writerow-put-a-comma-after-each-character
• https://stackoverflow.com/questions/466345/converting-string-into-datetime
• https://stackoverflow.com/questions/4142151/how-to-import-the-class-within-the-same-directory-or-sub-directory
• https://stackoverflow.com/questions/4906977/how-do-i-access-environment-variables-from-python
• https://stackoverflow.com/questions/44778/how-would-you-make-a-comma-separated-string-from-a-list-of-strings
• https://stackoverflow.com/questions/44505457/checking-if-an-environment-variable-exists-and-is-set-to-true
• https://stackoverflow.com/questions/3682748/converting-unix-timestamp-string-to-readable-date
• https://stackoverflow.com/questions/21787496/converting-epoch-time-with-milliseconds-to-datetime
• https://stackoverflow.com/questions/316238/python-float-to-decimal-conversion
• https://stackoverflow.com/questions/1252357/is-there-an-object-unique-identifier-in-python
• https://stackoverflow.com/questions/5998245/get-current-time-in-milliseconds-in-python
• https://qiita.com/alchemist/items/c75174c41b0bcd31ecc6
• https://docs.python.org/3/tutorial/datastructures.html
• https://stackoverflow.com/questions/15768757/how-to-construct-a-set-out-of-list-items-in-python
• https://stackoverflow.com/questions/3437059/does-python-have-a-string-contains-substring-method
• https://stackoverflow.com/questions/6797984/how-to-convert-string-to-lowercase-in-python
• https://www.tutorialspoint.com/python/string_replace.htm
• https://stackoverflow.com/questions/761804/how-do-i-trim-whitespace-from-a-python-string
• https://docs.python.org/3.6/library/re.html#module-re
• https://qiita.com/y__sama/items/a2c458de97c4aa5a98e7
• https://www.tutorialspoint.com/python/python_numbers.htm
• https://stackoverflow.com/questions/24130745/convert-generator-object-to-list-for-debugging
• https://stackoverflow.com/questions/11743019/convert-python-datetime-to-epoch-with-strftime
• https://qiita.com/miyagaw61/items/0e8cc40d95bf6236729e

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