String in Python
Contents
String in Python#
A string is a collection of characters/letters, in other terms characters are simple letters and symbols that are used to make a string and characters include alphanumeric letters, numbers and whitespace.
In Python, strings are written and enclosed between either single or double quotes. Once a string variable has been
created and declared, it is impossible to change its values, unless a new string is created. Assigning a string to a
variable is done with the variable name followed by an equal sign = and the string.
intf = 'FastEthernet1/0'
print(intf)
FastEthernet1/0
Using triple quotes allows the creation of multi-line strings. Any whitespace within the string will also be included.
intf_ip = """
interface FastEthernet1/0
ip address 172.16.10.1 255.255.255.0
"""
print(intf_ip)
interface FastEthernet1/0
ip address 172.16.10.1 255.255.255.0
String Slicing#
There are various ways of manipulating a string using the slicing method, which works the same way that it does for a list.
intf = "interface FastEthernet1/0"
print(intf[0:9])
print(intf[10:])
interface
FastEthernet1/0
String Concatenation#
Strings allow concatenation for joining two strings into one or combine text with numbers.
Addition#
The + operator allows the user to put together two strings to come up with the third string.
intf = "interface"
intf_name = "FastEthernet0/1"
cmd = intf + " " + intf_name
print(cmd)
interface FastEthernet0/1
String Replication#
The nice thing about the multiplication operator * is that it’s also possible to use it on strings. For example,
to print a line of 60 dashes.
print("#" * 60)
############################################################
String Method#
Python has a set of built-in methods that you can use on strings.
Note
All string methods return new values. They do not change the original string.
# methods lower(), upper(), isdigit() and startswith()
vendor = "Samsung Ltd"
print(vendor.upper()) # Upper case
print(vendor.lower()) # lower case
SAMSUNG LTD
samsung ltd
strip() method#
You will find that the strip(), split() and splitlines() methods are especially useful when parsing or
manipulating text from a network device.
If you have the value of text ” 192.168.10.10 ” including the whitespace. The methods startswith() or endswith()
do not work because of the spaces. To remove the whitespace from this value, strip() method is used.
>>> ipaddr = ' 192.168.10.10 '
>>> ipaddr
' 192.168.10.10 '
>>> ipaddr.strip()
'192.168.10.10'
split() method#
Return a list of the substrings in the string:
motd_banner = 'This router is configured by Python script'
print(motd_banner.split()) # The result is a list
['This', 'router', 'is', 'configured', 'by', 'Python', 'script']
splitlines() method#
The splitlines() method splits a string into a list, where each line is a list item. The splitting is done at line breaks:
my_text = """Welcome to the Network Automation.
Thank you Python...
"""
text = my_text.splitlines()
print(text)
['Welcome to the Network Automation.', 'Thank you Python...']
To get a full listing of the methods and attributes that you can access, you can use Python’s built-in dir() Function
To learn how to use a given method that you see in the output of a dir(). Use the built-in help() function, the following examples show the way you can use help() method:
[$] <> python Python 3.10.7 (tags/v3.10.7:6cc6b13, Sep 5 2022, 14:08:36) >>> help(str.split) Help on method_descriptor: split(self, /, sep=None, maxsplit=-1) Return a list of the substrings in the string, using sep as the separator string. Output is Omitted
join() method#
The join() method takes all items in an iterable (e.g. list/tuple) and joins them into one string. A string must be
specified as the separator.
motd_str = ["Python", "Network", "Automation"]
motd_banner = "#".join(motd_str)
print(motd_banner)
Python#Network#Automation
Escape Sequences#
There is a special character, that is understood by the compiler as commands other than characters. All these characters
are known as the escape sequences. The special characters are marked with a backslash \. The backslash \ would,
therefore, indicate that the character that is going to be printed next is a special one that should initiate some
instructions. Below are examples of special characters that are used with strings:
\nNewline\tHorizontal tab\rCarriage return\bBackslash
Raw String#
To avoid special processing on a string such as escape sequences. Specify a raw string by prefixing r or R to the
string.
string = r"newline is indicated by \n."
print(string)
newline is indicated by \n.
String Formatting#
String formatting or string substitution is where you have a string that you would like to insert into another string.
Python has three different ways to accomplish string formatting:
Using the
%MethodUsing
format()Using formatted string literals (
f-strings)
Formatting string using %#
Using the % method is Python’s oldest method of string formatting. The most common use of using the % sign is when
you would use %s, which means converting any Python object to a string using str().
ip_addr = "172.16.10.1"
print("IP Address: %s" %ip_addr) # insert string
IP Address: 172.16.10.1
ios_version = 15
print("iOS Version: %i" %ios_version) # insert integer
iOS Version: 15
ip_addr = "172.16.10.1"
mask = "255.255.255.0"
# formatting with multiple variables
print("IP Address: %s. Mask: %s" %(ip_addr, mask))
IP Address: 172.16.10.1. Mask: 255.255.255.0
To pass in multiple items, you use the % sign followed by a tuple of the items to insert.
Formatting string using format()#
The format() method formats the specified value(s) and insert them inside the string’s placeholder. The placeholder
is defined using curly brackets: {}. The format() method returns the formatted string.
ip_addr = "172.16.10.1"
mask = "255.255.255.0"
print("IP Address: {} Mask: {}".format(ip_addr, mask))
IP Address: 172.16.10.1 Mask: 255.255.255.0
This example uses positional arguments. Python looks for two instances of {} and will insert the variables accordingly.
ip_addr = "172.16.10.1"
subnet = "255.255.255.0"
print("IP Address: {ip} Mask: {mask}".format(ip=ip_addr, mask=subnet))
IP Address: 172.16.10.1 Mask: 255.255.255.0
Using dictionary
credentials = {"user": "admin", "pass": "P@ssW0rd"}
print("Username: {user} \nPassword: {pass}".format(**credentials))
Username: admin
Password: P@ssW0rd
A common coding style when working with format() is to create a formatted string and save it to a variable to
be used later.
description = """
Device: {}
IP: {}
"""
output = description.format('SW-01', '10.10.10.10')
print(output)
Device: SW-01
IP: 10.10.10.10
Formatting string with f-strings#
Formatted string literals or f-strings are strings that have an f at the beginning and curly braces inside of them
that contain expressions, much like the ones you saw in the previous section.
ip_addr = '172.16.10.1'
mask = "255.255.255.0"
print(f'IP Address {ip_addr} Mask: {mask}')
IP Address 172.16.10.1 Mask: 255.255.255.0
The f-string can do things that neither %s nor format() can do. Because f-strings are evaluated at runtime,
you can put any valid Python expression inside of them.
age = 25
print(f"{age + 5}")
30
One more example as below:
name = 'Ali'
print(f'{name.lower()}') # call a method or function
ali
Note
The f-string was added in Python 3.6.