Page 2 of 12

Python CSV

Working with csv files in Python

csv3

This article explains how to load and parse a CSV file in Python.

First of all, what is a CSV ?
CSV (Comma Separated Values) is a simple file format used to store tabular data, such as a spreadsheet or database. A CSV file stores tabular data (numbers and text) in plain text. Each line of the file is a data record. Each record consists of one or more fields, separated by commas. The use of the comma as a field separator is the source of the name for this file format.

For working CSV files in python, there is an inbuilt module called csv.

Reading a CSV file

# importing csv module 
import csv 

# csv file name 
filename = "aapl.csv"

# initializing the titles and rows list 
fields = [] 
rows = [] 

# reading csv file 
with open(filename, 'r') as csvfile: 
	# creating a csv reader object 
	csvreader = csv.reader(csvfile) 
	
	# extracting field names through first row 
	fields = next(csvreader) 

	# extracting each data row one by one 
	for row in csvreader: 
		rows.append(row) 

	# get total number of rows 
	print("Total no. of rows: %d"%(csvreader.line_num)) 

# printing the field names 
print('Field names are:' + ', '.join(field for field in fields)) 

# printing first 5 rows 
print('\nFirst 5 rows are:\n') 
for row in rows[:5]: 
	# parsing each column of a row 
	for col in row: 
		print("%10s"%col), 
	print('\n') 

The output of above program looks like this:

csv1

The above example uses a CSV file aapl.csv which can be downloaded from here.
Run this program with the aapl.csv file in same directory.

Let us try to understand this piece of code.

  • with open(filename, ‘r’) as csvfile: csvreader = csv.reader(csvfile)Here, we first open the CSV file in READ mode. The file object is named as csvfile. The file object is converted to csv.reader object. We save the csv.reader object as csvreader.
  • fields = csvreader.next()csvreader is an iterable object. Hence, .next() method returns the current row and advances the iterator to the next row. Since the first row of our csv file contains the headers (or field names), we save them in a list called fields.
  • for row in csvreader: rows.append(row)Now, we iterate through remaining rows using a for loop. Each row is appended to a list called rows. If you try to print each row, one can find that row is nothing but a list containing all the field values.
  • print(“Total no. of rows: %d”%(csvreader.line_num))csvreader.line_num is nothing but a counter which returns the number of rows which have been iterated.

Writing to a CSV file

# importing the csv module 
import csv 

# field names 
fields = ['Name', 'Branch', 'Year', 'CGPA'] 

# data rows of csv file 
rows = [ ['Nikhil', 'COE', '2', '9.0'], 
		['Sanchit', 'COE', '2', '9.1'], 
		['Aditya', 'IT', '2', '9.3'], 
		['Sagar', 'SE', '1', '9.5'], 
		['Prateek', 'MCE', '3', '7.8'], 
		['Sahil', 'EP', '2', '9.1']] 

# name of csv file 
filename = "university_records.csv"

# writing to csv file 
with open(filename, 'w') as csvfile: 
	# creating a csv writer object 
	csvwriter = csv.writer(csvfile) 
	
	# writing the fields 
	csvwriter.writerow(fields) 
	
	# writing the data rows 
	csvwriter.writerows(rows)

Let us try to understand the above code in pieces.

  • fields and rows have been already defined. fields is a list containing all the field names. rows is a list of lists. Each row is a list containing the field values of that row.
  • with open(filename, ‘w’) as csvfile: csvwriter = csv.writer(csvfile)Here, we first open the CSV file in WRITE mode. The file object is named as csvfile. The file object is converted to csv.writer object. We save the csv.writer object as csvwriter.
  • csvwriter.writerow(fields)Now we use writerow method to write the first row which is nothing but the field names.
  • csvwriter.writerows(rows)We use writerows method to write multiple rows at once.

Writing a dictionary to a CSV file

# importing the csv module 
import csv 

# my data rows as dictionary objects 
mydict =[{'branch': 'COE', 'cgpa': '9.0', 'name': 'Nikhil', 'year': '2'}, 
		{'branch': 'COE', 'cgpa': '9.1', 'name': 'Sanchit', 'year': '2'}, 
		{'branch': 'IT', 'cgpa': '9.3', 'name': 'Aditya', 'year': '2'}, 
		{'branch': 'SE', 'cgpa': '9.5', 'name': 'Sagar', 'year': '1'}, 
		{'branch': 'MCE', 'cgpa': '7.8', 'name': 'Prateek', 'year': '3'}, 
		{'branch': 'EP', 'cgpa': '9.1', 'name': 'Sahil', 'year': '2'}] 

# field names 
fields = ['name', 'branch', 'year', 'cgpa'] 

# name of csv file 
filename = "university_records.csv"

# writing to csv file 
with open(filename, 'w') as csvfile: 
	# creating a csv dict writer object 
	writer = csv.DictWriter(csvfile, fieldnames = fields) 
	
	# writing headers (field names) 
	writer.writeheader() 
	
	# writing data rows 
	writer.writerows(mydict) 

In this example, we write a dictionary mydict to a CSV file.

  • with open(filename, ‘w’) as csvfile: writer = csv.DictWriter(csvfile, fieldnames = fields)Here, the file object (csvfile) is converted to a DictWriter object.
    Here, we specify the fieldnames as an argument.
  • writer.writeheader()writeheader method simply writes the first row of your csv file using the pre-specified fieldnames.
  • writer.writerows(mydict)writerows method simply writes all the rows but in each row, it writes only the values(not keys).

So, in the end, our CSV file looks like this:

csv2

Important Points:

  • In csv modules, an optional dialect parameter can be given which is used to define a set of parameters specific to a particular CSV format. By default, csv module uses excel dialect which makes them compatible with excel spreadsheets. You can define your own dialect using register_dialect method.
    Here is an example:
     csv.register_dialect(
    'mydialect',
    delimiter = ',',
    quotechar = '"',
    doublequote = True,
    skipinitialspace = True,
    lineterminator = '\r\n',
    quoting = csv.QUOTE_MINIMAL)

Now, while defining a csv.reader or csv.writer object, we can specify the dialect like
this:

csvreader = csv.reader(csvfile, dialect='mydialect')
  • Now, consider that a CSV file looks like this in plain-text:
    We notice that the delimiter is not a comma but a semi-colon. Also, the rows are separated by two newlines instead of one. In such cases, we can specify the delimiter and line terminator as follows:csvreader = csv.reader(csvfile, delimiter = ';', lineterminator = '\n\n')

Python JSON

Introduction of JSON in Python :
The full-form of JSON is JavaScript Object Notation. It means that a script (executable) file which is made of text in a programming language, is used to store and transfer the data. Python supports JSON through a built-in package called json. To use this feature, we import the json package in Python script. The text in JSON is done through quoted string which contains value in key-value mapping within { }. It is similar to the dictionary in Python. JSON shows an API similar to users of Standard Library marshal and pickle modules and Python natively supports JSON features.

Python program showing
 use of json package
 import json 
 {key:value mapping}
 a ={"name":"John", 
 "age":31, 
     "Salary":25000} 
 conversion to JSON done by dumps() function
 b = json.dumps(a) 
 printing the output
 print(b) 

Output:

{"age": 31, "Salary": 25000, "name": "John"}

As you can see, JSON supports primitive types, like strings and numbers, as well as nested lists, tuples and objects.

Python program showing that
 json support different primitive
 types
 import json 
 list conversion to Array
 print(json.dumps(['Welcome', "to", "GeeksforGeeks"])) 
 tuple conversion to Array
 print(json.dumps(("Welcome", "to", "GeeksforGeeks"))) 
 string conversion to String
 print(json.dumps("Hi")) 
 int conversion to Number
 print(json.dumps(123)) 
 float conversion to Number
 print(json.dumps(23.572)) 
 Boolean conversion to their respective values
 print(json.dumps(True)) 
 print(json.dumps(False)) 
 None value to null
 print(json.dumps(None)) 

Output:

["Welcome", "to", "GeeksforGeeks"]
["Welcome", "to", "GeeksforGeeks"]
"Hi"
123
23.572
true
false
null

Python Django

Django is a Python-based web framework which allows you to quickly create web application without all of the installation or dependency problems that you normally will find with other frameworks.
When you’re building a website, you always need a similar set of components: a way to handle user authentication (signing up, signing in, signing out), a management panel for your website, forms, a way to upload files, etc. Django gives you ready-made components to use.

django-basics

Why Django?

  • Django is a rapid web development framework that can be used to develop fully fleshed web applications in a short period of time.
  • It’s very easy to switch database in Django framework.
  • It has built-in admin interface which makes easy to work with it.
  • Django is fully functional framework that requires nothing else.
  • It has thousands of additional packages available.
  • It is very scalable. For more visit When to Use Django? Comparison with other Development Stacks ?

Django architecture

Django is based on MVT (Model-View-Template) architecture. MVT is a software design pattern for developing a web application.

MVT Structure has the following three parts –

Model: Model is going to act as the interface of your data. It is responsible for maintaining data. It is the logical data structure behind the entire application and is represented by a database (generally relational databases such as MySql, Postgres).

Python Pandas

Pandas is an open-source library that is made mainly for working with relational or labeled data both easily and intuitively. It provides various data structures and operations for manipulating numerical data and time series. This library is built on the top of the NumPy library. Pandas is fast and it has high-performance & productivity for users.

Table of Content

  • History
  • Advantages
  • Getting Started
    • Series
    • DataFrame
  • Why Pandas is used for Data Science

History

Pandas was initially developed by Wes McKinney in 2008 while he was working at AQR Capital Management. He convinced the AQR to allow him to open source the Pandas. Another AQR employee, Chang She, joined as the second major contributor to the library in 2012. Over the time many versions of pandas have been released. The latest version of the pandas is 1.0.1

Advantages

  • Fast and efficient for manipulating and analyzing data.
  • Data from different file objects can be loaded.
  • Easy handling of missing data (represented as NaN) in floating point as well as non-floating point data
  • Size mutability: columns can be inserted and deleted from DataFrame and higher dimensional objects
  • Data set merging and joining.
  • Flexible reshaping and pivoting of data sets
  • Provides time-series functionality.
  • Powerful group by functionality for performing split-apply-combine operations on data sets.

Getting Started

After the pandas has been installed into the system, you need to import the library. This module is generally imported as –

import pandas as pd

Here, pd is referred to as an alias to the Pandas. However, it is not necessary to import the library using alias, it just helps in writing less amount of code everytime a method or property is called.

Python NumPy

Numpy is a general-purpose array-processing package. It provides a high-performance multidimensional array object, and tools for working with these arrays. It is the fundamental package for scientific computing with Python.
Besides its obvious scientific uses, Numpy can also be used as an efficient multi-dimensional container of generic data.

Arrays in Numpy

Array in Numpy is a table of elements (usually numbers), all of the same type, indexed by a tuple of positive integers. In Numpy, number of dimensions of the array is called rank of the array.A tuple of integers giving the size of the array along each dimension is known as shape of the array. An array class in Numpy is called as ndarray. Elements in Numpy arrays are accessed by using square brackets and can be initialized by using nested Python Lists.

Creating a Numpy Array
Arrays in Numpy can be created by multiple ways, with various number of Ranks, defining the size of the Array. Arrays can also be created with the use of various data types such as lists, tuples, etc. The type of the resultant array is deduced from the type of the elements in the sequences.
Note: Type of array can be explicitly defined while creating the array.

<code># Python program for</code>
<code># Creation of Arrays</code>
<code>import</code> <code>numpy as np</code>
&nbsp;
<code># Creating a rank 1 Array</code>
<code>arr =</code> <code>np.array([1, 2, 3])</code>
<code>print("Array with Rank 1: \n",arr)</code>
&nbsp;
<code># Creating a rank 2 Array</code>
<code>arr =</code> <code>np.array([[1, 2, 3],</code>
<code>[4, 5, 6]])</code>
<code>print("Array with Rank 2: \n", arr)</code>
&nbsp;
<code># Creating an array from tuple</code>
<code>arr =</code> <code>np.array((1, 3, 2))</code>
<code>print("\nArray created using "</code>
<code>"passed tuple:\n", arr)</code>

Output:

Array with Rank 1: 
 [1 2 3]
Array with Rank 2: 
 [[1 2 3]
 [4 5 6]]

Array created using passed tuple:
 [1 3 2]

 
Accessing the array Index
In a numpy array, indexing or accessing the array index can be done in multiple ways. To print a range of an array, slicing is done. Slicing of an array is defining a range in a new array which is used to print a range of elements from the original array. Since, sliced array holds a range of elements of the original array, modifying content with the help of sliced array modifies the original array content.

<code># Python program to demonstrate</code>
<code># indexing in numpy array</code>
<code>import</code> <code>numpy as np</code>
&nbsp;
<code># Initial Array</code>
<code>arr =</code> <code>np.array([[-1, 2, 0, 4],</code>
<code>[4, -0.5, 6, 0],</code>
<code>[2.6, 0, 7, 8],</code>
<code>[3, -7, 4, 2.0]])</code>
<code>print("Initial Array: ")</code>
<code>print(arr)</code>
&nbsp;
<code># Printing a range of Array</code>
<code># with the use of slicing method</code>
<code>sliced_arr =</code> <code>arr[:2, ::2]</code>
<code>print</code> <code>("Array with first 2 rows and"</code>
<code>" alternate columns(0 and 2):\n", sliced_arr)</code>
&nbsp;
<code># Printing elements at</code>
<code># specific Indices</code>
<code>Index_arr =</code> <code>arr[[1, 1, 0, 3], </code>
<code>[3, 2, 1, 0]]</code>
<code>print</code> <code>("\nElements at indices (1, 3), "</code>
<code>"(1, 2), (0, 1), (3, 0):\n", Index_arr)</code>

Output:

Initial Array: 
[[-1.   2.   0.   4. ]
 [ 4.  -0.5  6.   0. ]
 [ 2.6  0.   7.   8. ]
 [ 3.  -7.   4.   2. ]]
Array with first 2 rows and alternate columns(0 and 2):
 [[-1.  0.]
 [ 4.  6.]]

Elements at indices (1, 3), (1, 2), (0, 1), (3, 0):
 [ 0. 54.  2.  3.]

Python Collections

Counter is a container included in the collections module. Now you all must be wondering what is a container. Don’t worry first let’s discuss about the container.

What is Container?

Containers are objects that hold objects. They provide a way to access the contained objects and iterate over them. Examples of built in containers are Tuple, list, and dictionary. Others are included in Collections module.

A Counter is a subclass of dict. Therefore it is an unordered collection where elements and their respective count are stored as a dictionary. This is equivalent to a bag or multiset of other languages.

Syntax :

class collections.Counter([iterable-or-mapping])

Initialization :
The constructor of counter can be called in any one of the following ways :

With sequence of itemsWith dictionary containing keys and countsWith keyword arguments mapping string names to counts.

OrderedDict in Python

An OrderedDict is a dictionary subclass that remembers the order that keys were first inserted. The only difference between dict() and OrderedDict() is that:

OrderedDict preserves the order in which the keys are inserted. A regular dict doesn’t track the insertion order, and iterating it gives the values in an arbitrary order. By contrast, the order the items are inserted is remembered by OrderedDict.

A Python program to demonstrate working of OrderedDict
 from collections import OrderedDict 
 print("This is a Dict:\n") 
 d = {} 
 d['a'] = 1
 d['b'] = 2
 d['c'] = 3
 d['d'] = 4
 for key, value in d.items(): 
     print(key, value) 
 print("\nThis is an Ordered Dict:\n") 
 od = OrderedDict() 
 od['a'] = 1
 od['b'] = 2
 od['c'] = 3
 od['d'] = 4
 for key, value in od.items(): 
     print(key, value) 

Output:

This is a Dict:
('a', 1)
('c', 3)
('b', 2)
('d', 4)

This is an Ordered Dict:
('a', 1)
('b', 2)
('c', 3)
('d', 4)

Important Points:

  1. Key value Change: If the value of a certain key is changed, the position of the key remains unchanged in OrderedDict.
A Python program to demonstrate working of key
 value change in OrderedDict
 from collections import OrderedDict 
 print("Before:\n") 
 od = OrderedDict() 
 od['a'] = 1
 od['b'] = 2
 od['c'] = 3
 od['d'] = 4
 for key, value in od.items(): 
     print(key, value) 
 print("\nAfter:\n") 
 od['c'] = 5
 for key, value in od.items(): 
     print(key, value) 
Before: ('a', 1) ('b', 2) ('c', 3) ('d', 4) 
After: ('a', 1) ('b', 2) ('c', 5) ('d', 4)

Other Considerations:

  • Ordered dict in Python version 2.7 consumes more memory than normal dict. This is due to the underlying Doubly Linked List implementation for keeping the order. In Python 2.7 Ordered Dict is not dict subclass, it’s a specialized container from collections module.
  • Starting from Python 3.7, insertion order of Python dictionaries is guaranteed.
  • Ordered Dict can be used as a stack with the help of popitem function. Try implementing LRU cache with Ordered Dict.

Python Regex

Regular Expression in Python with Examples | Set 1

Last Updated: 19-10-2020

Module Regular Expressions(RE) specifies a set of strings(pattern) that matches it. 
To understand the RE analogy, MetaCharacters are useful, important and will be used in functions of module re. 
There are a total of 14 metacharacters and will be discussed as they follow into functions: 

\   Used to drop the special meaning of character
    following it (discussed below)
[]  Represent a character class
^   Matches the beginning
$   Matches the end
.   Matches any character except newline
?   Matches zero or one occurrence.
|   Means OR (Matches with any of the characters
    separated by it.
*   Any number of occurrences (including 0 occurrences)
+   One or more occurrences
{}  Indicate number of occurrences of a preceding RE 
    to match.
()  Enclose a group of REs



  • Function compile() 
    Regular expressions are compiled into pattern objects, which have methods for various operations such as searching for pattern matches or performing string substitutions. 
     
#Module Regular Expression is imported using <strong>import</strong>().
 import re 
# compile() creates regular expression character class [a-e],
# which is equivalent to [abcde].
# class [abcde] will match with string with 'a', 'b', 'c', 'd', 'e'.
 p = re.compile('[a-e]') 
# findall() searches for the Regular Expression and return a list upon finding
 print(p.findall("Aye, said Mr. Gibenson Stark")) 

Output: 

['e', 'a', 'd', 'b', 'e', 'a']




Understanding the Output: 
First occurrence is ‘e’ in “Aye” and not ‘A’, as it being Case Sensitive. 
Next Occurrence is ‘a’ in “said”, then ‘d’ in “said”, followed by ‘b’ and ‘e’ in “Gibenson”, the Last ‘a’ matches with “Stark”.
Metacharacter backslash ‘\’ has a very important role as it signals various sequences. If the backslash is to be used without its special meaning as metacharacter, use’\\’

\d   Matches any decimal digit, this is equivalent
     to the set class [0-9].
\D   Matches any non-digit character.
\s   Matches any whitespace character.
\S   Matches any non-whitespace character
\w   Matches any alphanumeric character, this is
     equivalent to the class [a-zA-Z0-9_].
\W   Matches any non-alphanumeric character. 

File handling

Python too supports file handling and allows users to handle files i.e., to read and write files, along with many other file handling options, to operate on files. The concept of file handling has stretched over various other languages, but the implementation is either complicated or lengthy, but alike other concepts of Python, this concept here is also easy and short. Python treats file differently as text or binary and this is important. Each line of code includes a sequence of characters and they form text file. Each line of a file is terminated with a special character, called the EOL or End of Line characters like comma {,} or newline character. It ends the current line and tells the interpreter a new one has begun. Let’s start with Reading and Writing files.

Working of open() function

We use open () function in Python to open a file in read or write mode. As explained above, open ( ) will return a file object. To return a file object we use open() function along with two arguments, that accepts file name and the mode, whether to read or write. So, the syntax being: open(filename, mode). There are three kinds of mode, that Python provides and how files can be opened:

  •  r “, for reading.
  •  w “, for writing.
  •  a “, for appending.
  •  r+ “, for both reading and writing

One must keep in mind that the mode argument is not mandatory. If not passed, then Python will assume it to be “ r ” by default. Let’s look at this program and try to analyze how the read mode works:

#a file named "thanh", will be opened with the reading mode.
 file = open('thanh.txt', 'r') 
 This will print every line one by one in the file
 for each in file: 
     print (each) 

Working of read() mode

There is more than one way to read a file in Python. If you need to extract a string that contains all characters in the file then we can use file.read(). The full code would work like this:

#Python code to illustrate read() mode
 file = open("file.text", "r") 
 print file.read() 

Creating a file using write() mode

Let’s see how to create a file and how write mode works:
To manipulate the file, write the following in your Python environment:

#Python code to create a file
 file = open('geek.txt','w') 
 file.write("This is the write command") 
 file.write("It allows us to write in a particular file") 
 file.close() 

Exception Handling

Error in Python can be of two types i.e. Syntax errors and Exceptions. Errors are the problems in a program due to which the program will stop the execution. On the other hand, exceptions are raised when some internal events occur which changes the normal flow of the program.

The difference between Syntax Error and Exceptions

Syntax Error: As the name suggest this error is caused by wrong syntax in the code. It leads to the termination of the program.

#initialize the amount variable
 amount = 10000
# check that You are eligible to
# purchase Dsa Self Paced or not
 if(amount>2999) 
     print("You are eligible to purchase Dsa Self Paced") 

Output:

Exceptions: Exceptions are raised when the program is syntactically correct but the code resulted in an error. This error does not stop the execution of the program, however, it changes the normal flow of the program.

#initialize the amount variable
 marks = 10000
# perform division with 0
 a = marks / 0
 print(a) 

Output:

In the above example raised the ZeroDivisionError as we are trying to divide a number by 0.

Tiếng Việt

Ngoại lệ (Exception) trong Python

Ngoại lệ (Exception) là lỗi xảy ra trong quá trình thực thi một chương trình. Khi nó xảy ra, Python tạo ra một exception để xử lý vấn đề đó tránh cho ứng dụng hay server của bạn bị crash.

Ngoại lệ có thể là bất kỳ điều kiện bất thường nào trong chương trình phá vỡ luồng thực thi chương trình đó. Bất cứ khi nào một ngoại lệ xuất hiện, chương trình sẽ ngừng thực thi, chuyển qua quá trình gọi và in ra lỗi đến khi nó được xử lý.

Xử lý ngoại lệ trong Python

Trong Python, các ngoại lệ có thể được xử lý bằng khối lệnh try…except. Nghĩa là mình nói với máy tính là chạy đoạn code này đi (try)m trừ phi(except) điều này xảy ra.

Phần thân của try sẽ gồm code có thể tạo ra exception, nếu một exception được tạo ra, tất cả câu lệnh trong khối sẽ bị bỏ qua.

Mặt khác, phần thân của except được gọi bởi exception handler, vì nó được dùng để bắt lỗi. Khối except sẽ thực hiện khi lỗi được tạo ra, không thì sẽ được bỏ qua. Bạn có thể dùng exception có sẵn trong Thư viện chuẩn Python.

Ví dụ:

# import module sys để gọi ra các ngoại lệ
import sys

randomList = ['a', 0, 2]

for nhap in randomList:
try:
print("Phần tử:", nhap)
r = 1/int(nhap)
break
except:
print("Có ngoại lệ ",sys.exc_info()[0]," xảy ra.")
print("Nhập phần tử tiếp theo")
print()
print("Kết quả với phần tử",nhap,"là:",r)

Kết quả đưa ra màn hình như sau:

Phần tử: a
Có ngoại lệ <class 'ValueError'> xảy ra.
Nhập phần tử tiếp theo

Phần tử: 0
Có ngoại lệ <class 'ZeroDivisionError'> xảy ra.
Nhập phần tử tiếp theo

Phần tử: 2
Kết quả với phần tử 2 là: 0.5

Ở ví dụ này, chương trình sẽ thực thi đến khi có chính xác số được nhập là số nguyên.

Nếu không có ngoại lệ xảy ra, khối except sẽ được bỏ qua và chương trình theo luồng bình thường, nhưng nếu có bất cứ ngoại lệ nào nó sẽ bị chặn lại bởi except.

Chương trình cũng in tên của exception bằng hàm ex_info() bên trong module sys, kết quả trả về là giá trị ‘a’ gây ra ValueError và 0 gây ra ZeroDivisionError.

Nếu bạn không dùng except, thì nếu bạn nhập giá trị “a”, hoặc số “0”. Chương trình sẽ dừng lại và báo lỗi, nhưng nếu dùng “try..except”, thì chương trinh của bạn sẽ tiếp tục chạy và những trường hợp lỗi sẽ được thông báo. Điều này giống như là khi nhập password sai được phép nhập lại vậy, nếu không dùng try except thì máy tính bạn treo luôn ngay khi bạn nhập giá trị sai.

Xử lý cụ thể một ngoại lệ

Ở ví dụ trên, không có một ngoại lệ cụ thể nào được đề cập đến trong mệnh đề except nên khi chương trình gặp ngoại lệ (dù là bất kì exception nào) thì chúng đều được xử lý theo một cách.

Ngoài cách đó ra, ta có thể chỉ định các ngoại lệ cụ thể cho khối lệnh except.

Cú pháp như sau:

try:
# khối code lệnh try
except exceptionName:
# khối code lệnh except

Tham số:

  • exceptionName là tên của các exception bạn nghĩ có khả năng xảy ra.

Một mệnh đề try có thể có nhiều mệnh đề except để xử lý chúng khác nhau.

Nếu khối lệnh trong try có 1 lỗi gì đó xảy ra thì chương trình sẽ tìm đến các except phía dưới, except nào thỏa mãn thì nó sẽ thực thi code trong khối except đó.

try :
# khối code lệnh try

except ValueError:
# code xử lý ValueError

except RuntimeError:
# code xử lý RuntimeError

Bạn cũng có thể bắt nhiều exception trên một lần khai báo except bằng việc đặt các ngoại lệ cách nhau bởi một dấu phẩy ‘,’

try :
# khối code lệnh try

except (TypeError, ZeroDivisionError):
# code xử lý nhiều ngoại lệ
# TypeError, ZeroDivisionError

Xây dựng một Exception

Xây dựng một exception bằng cách sử dụng raise là một cách khác để xử lí ngoại lệ trong Python. Trong trường hợp này, bạn có thể tạo exception của riêng mình – đó là exception được nêu ra khi vấn đề nằm bên ngoài phạm vi của dự kiến lỗi xảy ra.

Ví dụ:

try:
x = input('Nhập một số trong khoảng 1-10: ')
if x<1 or x>10:
raise Exception
print 'Bạn vừa nhập một số hợp lệ :D'

except:
print 'Số bạn vừa nhập nằm ngoài khoảng cho phép mất rồi!'

Trong ví dụ này, nếu bạn nhập một số bên ngoài các phạm vi cho phép, các lệnh print trong các except block sẽ được thực hiện.

Module traceback

Module traceback là cách khác để xử lí exception trong Python. Về cơ bản nó được sử dụng để in ra dấu vết của chương trình sau khi exception xảy ra.

Traceback bao gồm thông báo lỗi, số dòng gây ra lỗi và call stack của hàm gây ra lỗi.

>>> raise KeyboardInterrupt
Traceback (most recent call last):
File "<pyshell#0>", line 1, in <module>
raise KeyboardInterrupt
KeyboardInterrupt

>>> raise MemoryError("Đây là một tham số.")
Traceback (most recent call last):
File "<pyshell#1>", line 1, in <module>
raise MemoryError("Đây là một tham số.")
MemoryError: Đây là một tham số.

>>> try:
... a = int(input("Nhập một số nguyên dương: "))
... if a <= 0:
... raise ValueError("Số bạn vừa nhập không phải số nguyên dương.")
... except ValueError as ve:
... print(ve)
...
Nhập một số nguyên dương: -2
Số bạn vừa nhập không phải số nguyên dương.

Try…Finally

Try…finally là một cách khác để viết lệnh try trong Python.

Finally còn được gọi là mệnh đề clean-up/termination vì nó luôn luôn chạy bất kể có lỗi nào xảy ra trong block try.

Thường thì các câu lệnh trong finally được dùng để thực hiện công việc giải phóng tài nguyên.

Ví dụ cho các hoạt động của file để minh họa rõ về finally:

Sau khi thực hiện xong các thao tác với file trong Python thì bạn cần đóng nó lại. Đóng file để đảm bảo quy chế đóng mở và giải phóng bộ nhớ cho chương trình.

try:
f = open("test.txt",encoding = 'utf-8')
# thực hiện các thao tác với tệp
finally:
f.close()

Bằng cách này, ta có thể yên tâm file được đóng đúng ngay cả khi phát sinh ngoại lệ khiến chương trình dừng đột ngột.

Một ví dụ khác có ngoại lệ:

mauso = input("Bạn hãy nhập giá trị mẫu số: ")
try:
ketqua = 15/int(mauso)
print("Kết quả là:",ketqua)
finally:
print("Bạn đã nhập số không thể thực hiện phép tính.")

Finally luôn luôn chạy bất kể có lỗi xảy ra hay không.

Khi bạn nhập input là 5, chương trình trả về kết quả:

Bạn hãy nhập giá trị mẫu số: 5
Kết quả là: 3.0
Bạn đã nhập số không thể thực hiện phép tính.

Và khi input là 0, kết quả hiển thị:

Bạn hãy nhập giá trị mẫu số: 0
Bạn đã nhập số không thể thực hiện phép tính.


 

Python OOP

A class is a user-defined blueprint or prototype from which objects are created. Classes provide a means of bundling data and functionality together. Creating a new class creates a new type of object, allowing new instances of that type to be made. Each class instance can have attributes attached to it for maintaining its state. Class instances can also have methods (defined by its class) for modifying its state.
To understand the need for creating a class let’s consider an example, let’s say you wanted to track the number of dogs which may have different attributes like breed, age. If a list is used, the first element could be the dog’s breed while the second element could represent its age. Let’s suppose there are 100 different dogs, then how would you know which element is supposed to be which? What if you wanted to add other properties to these dogs? This lacks organization and it’s the exact need for classes.
Class creates a user-defined data structure, which holds its own data members and member functions, which can be accessed and used by creating an instance of that class. A class is like a blueprint for an object.
Some points on Python class:
Classes are created by keyword class.
Attributes are the variables that belong to class.
Attributes are always public and can be accessed using dot (.) operator. Eg.: Myclass.Myattribute


<strong>Class Definition Syntax:</strong> class ClassName:     
# Statement-1     
.     .     .     
# Statement-N 
<strong>Defining a class –</strong>

python class

Declaring Objects (Also called instantiating a class)
When an object of a class is created, the class is said to be instantiated. All the instances share the attributes and the behavior of the class. But the values of those attributes, i.e. the state are unique for each object. A single class may have any number of instances.
Example:

python declaring an object

Declaring an object


# Python program to
# demonstrate instantiating
# a class


classDog: 

# A simple class
# attribute
attr1 ="mamal"
attr2 ="dog"

# A sample method  
deffun(self): 
print("I'm a", self.attr1)
print("I'm a", self.attr2)

# Driver code
# Object instantiation
Rodger =Dog()

# Accessing class attributes
# and method through objects
print(Rodger.attr1)
Rodger.fun()

Output:
mamal I’m a mamal I’m a dog
In the above example, an object is created which is basically a dog named Rodger. This class only has two class attributes that tell us that Rodger is a dog and a mammal.
The self
Class methods must have an extra first parameter in method definition. We do not give a value for this parameter when we call the method, Python provides it.
If we have a method which takes no arguments, then we still have to have one argument.
This is similar to this pointer in C++ and this reference in Java.
When we call a method of this object as myobject.method(arg1, arg2), this is automatically converted by Python into MyClass.method(myobject, arg1, arg2) – this is all the special self is about.
__init__ method:
The __init__ method is similar to constructors in C++ and Java. Constructors are used to initialize the object’s state. Like methods, a constructor also contains a collection of statements(i.e. instructions) that are executed at the time of Object creation. It is run as soon as an object of a class is instantiated. The method is useful to do any initialization you want to do with your object.

# A Sample class with init method 
classPerson: 
# init method or constructor  
def__init__(self, name): 
self.name =name 
# Sample Method  
defsay_hi(self): 
print('Hello, my name is', self.name)
p =Person('Nikhil') 
p.say_hi() 

 
Output:
Hello, my name is Nikhil
Class and Instance Variables
Instance variables are for data unique to each instance and class variables are for attributes and methods shared by all instances of the class. Instance variables are variables whose value is assigned inside a constructor or method with self whereas class variables are variables whose value is assigned in the class.
Defining instance varibale using constructor.

# Python program to show that the variables with a value  
# assigned in the class declaration, are class variables and 
# variables inside methods and constructors are instance 
# variables. 

# Class for Dog 
classDog: 

# Class Variable 
animal ='dog'

# The init method or constructor 
def__init__(self, breed, color): 

# Instance Variable     
self.breed =breed
self.color =color        

# Objects of Dog class 
Rodger =Dog("Pug", "brown") 
Buzo =Dog("Bulldog", "black") 

print('Rodger details:')   
print('Rodger is a', Rodger.animal) 
print('Breed: ', Rodger.breed)
print('Color: ', Rodger.color)

print('\nBuzo details:')   
print('Buzo is a', Buzo.animal) 
print('Breed: ', Buzo.breed)
print('Color: ', Buzo.color)

# Class variables can be accessed using class 
# name also 
print("\nAccessing class variable using class name")
print(Dog.animal)        

Output:
Rodger details: Rodger is a dog Breed: Pug Color: brown Buzo details: Buzo is a dog Breed: Bulldog Color: black Accessing class variable using class name dog
Defining instance variable using the normal method.

# Python program to show that we can create  
# instance variables inside methods 

# Class for Dog 
classDog: 

# Class Variable 
animal ='dog'

# The init method or constructor 
def__init__(self, breed): 

# Instance Variable 
self.breed =breed             

# Adds an instance variable  
defsetColor(self, color): 
self.color =color 

# Retrieves instance variable     
defgetColor(self):     
returnself.color    

# Driver Code 
Rodger =Dog("pug") 
Rodger.setColor("brown") 
print(Rodger.getColor())  
Output:
brown