Python is a popular programming language that can be used to conduct almost any project. When you learn python, you may come up with different questions regarding various tasks such as file processing, list, dict usage, database, time, url, et al. In this tutorial, we give clean solutions to some of the most frequently problems you may encounter when you learn python.

1. File related questions

   How to check whether a file exists using Python?

   How to check whether a path is a file?

   How to make sure an directory exist?

   How to list all files of a directory in Python?

   How to read a file line by line into a list with Python?

   How to append a line to file in Python?

   How to remove a file in Python?

2. Dictionary Related Questions

   How to create a dictionary?

   d = {1:11, 2:22} or d = dict([(1,11), (2,22)])

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   d = {1:11, 2:22} or d = dict([(1,11), (2,22)])

   How to add key to a dictionary in Python?

   d[3] = 33

   1

   d[3] = 33

   Check if a given key already exists in a dictionary

   if 3 in d: print '3 exist'

   1 2	if 3 in d:     print '3 exist'

   Sort a Python dictionary by value

   Use operator

   import operator x ={1:10,3:12,4:3,2:20,0:5} print x {0: 5, 1: 10, 2: 20, 3: 12, 4: 3}import operator sorted_x = sorted(x.items(), key=operator.itemgetter(1)) print sorted_x [(4, 3), (0, 5), (1, 10), (3, 12), (2, 20)]

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   import operator   x ={1:10,3:12,4:3,2:20,0:5} print x {0: 5, 1: 10, 2: 20, 3: 12, 4: 3}import operator   sorted_x = sorted(x.items(), key=operator.itemgetter(1)) print sorted_x   [(4, 3), (0, 5), (1, 10), (3, 12), (2, 20)]

   Use Lambda

   sort_x = sorted(x.items(), key=lambda (k, v): v)

   1	sort_x = sorted(x.items(), key=lambda (k, v): v)

   how to merge two dictionaries in Python?

   x = {1:1, 2:2, 3:3} y = {1:2, 2:3, 4:5} z = x.copy() z.update(y) print z {1: 2, 2: 3, 3: 3, 4: 5}

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   x = {1:1, 2:2, 3:3} y = {1:2, 2:3, 4:5} z = x.copy() z.update(y) print z {1: 2, 2: 3, 3: 3, 4: 5}

3. List Related Questions:

   How to check if a list is empty?

   a = [] if not a: print"List is empty"

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   a = [] if not a:     print"List is empty"

   if len(a) == 0: print 'List is empty'

   1 2	if len(a) == 0:     print 'List is empty'

   What’s the difference between the list methods append() and extend()?

   x =[1,2,3] x.append([4,5]) print(x) 1, 2, 3, [4, 5]]

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   x =[1,2,3] x.append([4,5]) print(x) 1, 2, 3, [4, 5]]

   So append: Appends object at end.

   ---

   However, extend: Extends list by appending elements from the iterable.

   x =[1,2,3] x.extend([4,5]) print(x) [1, 2, 3, 4, 5]

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   x =[1,2,3] x.extend([4,5]) print(x) [1, 2, 3, 4, 5]

4. Time related Questions

   How to get current time in python?

   import datetime print datetime.datetime.now() 2015-07-09 14:01:12.947795

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   import datetime print datetime.datetime.now() 2015-07-09 14:01:12.947795

   How to convert String to time object?

   from datetime import datetime obj = datetime.strptime('Jun 1 2015 12:50PM', '%b %d %Y %I:%M%p')

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   from datetime import datetime obj = datetime.strptime('Jun 1 2015  12:50PM', '%b %d %Y %I:%M%p')

5. Database Related questions

   How do I connect to a MySQL Database in Python?

   Install MySQLdb from http://www.kitebird.com/articles/pydbapi.html, then

   import MySQLdb db = MySQLdb.connect(host="hostname", # usually localhost or 127.0.0.1 user="user", # your username passwd="111111", # your password db="test_db") # name of the data base cur = db.cursor() cur.execute("SELECT * FROM test_table") for row in cur.fetchall(): print row db.close()

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   import MySQLdb db = MySQLdb.connect(host="hostname",    # usually localhost or 127.0.0.1                      user="user",         # your username                      passwd="111111",  # your password                      db="test_db")        # name of the data base   cur = db.cursor() cur.execute("SELECT * FROM test_table") for row in cur.fetchall():     print row db.close()

   Use MySQL Connector/Python from MySQL: http://dev.mysql.com/downloads/connector/python/.
   
   import mysql.connector db = mysql.connector.connect(user='user', password='111111', host='127.0.0.1', database='test_db') try: cursor = db.cursor() cursor.execute(""" select * from test_table """) for row in cur.fetchall(): print row finally: db.close()

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   import mysql.connector db = mysql.connector.connect(user='user', password='111111',                               host='127.0.0.1',                               database='test_db') try:    cursor = db.cursor()    cursor.execute("""       select * from test_table    """)    for row in cur.fetchall():        print row   finally:     db.close()