Python

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with statement was introduced in Python 2.5 and is quite useful when you want to work with a resource and then want to dispose/close it when you are done.

The with statement is used for writing blocks of code when we want to call methods as defined by context.It allows to write block of code in a more concise way then
try finally block.The with statement is used to wrap the execution of a block with methods defined by a context manager

you use with statement as:

specify expression after with statement followed by target variable

with expression as variable

when the above statement is executed the __enter__ method of the expression result is called and the result is assigned to the given variable.

In following example we are creating an object of file using the with statement.The benefit of using with statement here is that the file object will be automatically closed after the block executes.Even if exception occurs  in the while block ,the file will still be closed.

import csv
with open('csvSample.csv') as file:
csv_values = csv.DictReader(file)
for row in csv_values:
print("%s,%s"%(row['column1'],row['column2']))

Without using with statement we will have to close the file explicitly.Same logic as the above sample can be written without using with statement as:

file = open('csvSample.csv')
csv_values = csv.DictReader(file)
for row in csv_values:
print("%s,%s"%(row['column1'],row['column2']))
file.close()

The above logic is not as concise as the previous one.Not only this if an exception happens file object will not be closed unlike the with statement.When we use with statement the file is guaranteed to be closed whether exception happens or not.

If you want to use your custom class in with statement then you need to define the enter and  exit methods:

class SampleClass():
def __init__(self, value):
# constructor logic
def __enter__(self):
# create logic
def __exit__(self, type, value, traceback):
#destroy logic

The above is just a pseudo code,but you can use it to define actual class which can be used in with statement.

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Determine variable type in Python

To get the type of a variable in python you can use the built in type() method.

x=1
print(type(x))

output:

<class ‘int’>

In the following code we are printing the value the data type of variable x.

x=1
print(type(x))
y=type(x)
if y==int:
print("variable is integer")
elif y==str: 
print("variable is string")
elif y==float:
print("variable is float")

If you execute the code above then the following will be printed:
variable is integer

try changing the value of x to different types such as string,float.To set float value you can assign x as:

x=1.1

Similarly to assign string to x:

x=”Hello!”

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Reading and writing csv files in Python

Python includes functionality in the form of modules.There is a library which is included as part of Python.If other features are required then there are extra libraries which can be downloaded.To work with CSV files in Python there is a CSV module.Using the csv module we can read and write csv files.

Reading CSV files

To read the contents of CSV file we can use the DictReader method of the csv module.We pass the file object and delimiter as arguments.Though delimiter is optional argument and is not required.We can specify any delimiter we want to use while reading the file depending on the file delimiter.

In the following example we are reading the contents of the csv file called csvSample.csv and printing the values of the different rows and columns:

import csv
with open('csvSample.csv') as f:
csv_values = csv.DictReader(f)
for row in csv_values:
print("%s,%s"%(row['column1'],row['column2']))

if required we can pass delimiter value in the DictReader method as:

csv_values = csv.DictReader(f,delimiter=',')

Here we are specifying “,” as delimeter but we can specify other delimeters as required.

Writing CSV files

We can write values to csv files using the CSV module as:

data = [['employee_name', 'employee_id', 'department'],
['Emp1', '1', 'City1'],
['Emp2', '2', 'City2'],
['Emp3', '3', 'City3']]

with open(path, "a") as csv_file:
writer = csv.writer(csv_file, delimiter=',')
for line in data:
writer.writerow(line)

 

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To work with files in Python os and shutil are useful modules.The os module consists of methods related to files and directories.For working with Files in Python you need to use the File object which is a built in object in Python.It consists of some useful methods

• Open() Used to open a file
• Read () Used to read contents of the file
• Write () Used for writing to a file
• Append () Used for appending new content

To open a file called sample.txt having the full path as “C:\sample.txt”, you will use the following code.We are opening the file in read mode by specifying “r” as the second argument to the open method:

a=open(r'C:\sample.txt',"r")

Similarly to open the file in write mode you will use the following:

a=open(r'C:\sample.txt',"w")

Similarly for appending to an existing file you will pass “a” as the second argument.

You can use the following sample to read and print the contents of a file:
a=open(r'I:\sample.txt',"r")
b=a.readlines()
print(b)

If you observe you can notice one important point above.We have used the character ‘r’ before string path argument in open method.This is required as “\” is a reseverd character in Python.If you want you can specify double slashed “\\” instead of character ‘r’ when specifying paths.

To list the contents of a directory you can use the os modules listdir method as:

print(os.listdir("I:\\"))

To move a file you can use shutil module’s move method:

shutil.move("C:\\sample.txt", "K:\\sample.log")

similarly to copy a file from one location to another you can use the copy method:

shutil.copy("C:\\sample.txt", "K:\\sample.log")

 

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