Unpacking arguments#
In Python the expressions *args and **kwargs allow for another
task - unpacking arguments.
So far we’ve called all functions manually. Hence, we passed on all relevant arguments.
In reality, it is usually necessary to pass data to function programmatically. And often data comes in the form of a Python object.
Unpacking positional arguments#
For example, when formatting strings you often need to pass multiple arguments
to format method. And often these arguments are already in list or tuple.
To pass them to format method you have to use indexes:
In [1]: items = [1, 2, 3]
In [2]: print('One: {}, Two: {}, Three: {}'.format(items[0], items[1], items[2]))
One: 1, Two: 2, Three: 3
Instead, you can take advantage of unpacking argument and do this:
In [4]: items = [1, 2, 3]
In [5]: print('One: {}, Two: {}, Three: {}'.format(*items))
One: 1, Two: 2, Three: 3
Another example is config_interface function (func_config_interface_unpacking.py file):
In [8]: def config_interface(intf_name, ip_address, mask):
..: interface = f'interface {intf_name}'
..: no_shut = 'no shutdown'
..: ip_addr = f'ip address {ip_address} {mask}'
..: result = [interface, no_shut, ip_addr]
..: return result
..:
Function expects such arguments:
intf_name - interface name
ip_address - IP address
mask - subnet mask
Function returns a list of strings to configure interface:
In [9]: config_interface('Fa0/1', '10.0.1.1', '255.255.255.0')
Out[9]: ['interface Fa0/1', 'no shutdown', 'ip address 10.0.1.1 255.255.255.0']
In [11]: config_interface('Fa0/10', '10.255.4.1', '255.255.255.0')
Out[11]: ['interface Fa0/10', 'no shutdown', 'ip address 10.255.4.1 255.255.255.0']
Suppose you call a function and pass it information that has been obtained from another source, for example from database. For example, interfaces_info list contains parameters for configuring interfaces:
In [14]: interfaces_info = [['Fa0/1', '10.0.1.1', '255.255.255.0'],
...: ['Fa0/2', '10.0.2.1', '255.255.255.0'],
...: ['Fa0/3', '10.0.3.1', '255.255.255.0'],
...: ['Fa0/4', '10.0.4.1', '255.255.255.0'],
...: ['Lo0', '10.0.0.1', '255.255.255.255']]
...:
If you go through list in the loop and pass nested list as a function argument, an error will occur:
In [15]: for info in interfaces_info:
...: print(config_interface(info))
...:
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-15-d34ced60c796> in <module>
1 for info in interfaces_info:
----> 2 print(config_interface(info))
3
TypeError: config_interface() missing 2 required positional arguments: 'ip_address' and 'mask'
Error is quite logical: function expects three arguments and it is
given 1 argument - a list. In such a situation it is necessary to unpack
arguments. Just add * before passing the list as an argument and there is no error anymore:
In [16]: for info in interfaces_info:
...: print(config_interface(*info))
...:
['interface Fa0/1', 'no shutdown', 'ip address 10.0.1.1 255.255.255.0']
['interface Fa0/2', 'no shutdown', 'ip address 10.0.2.1 255.255.255.0']
['interface Fa0/3', 'no shutdown', 'ip address 10.0.3.1 255.255.255.0']
['interface Fa0/4', 'no shutdown', 'ip address 10.0.4.1 255.255.255.0']
['interface Lo0', 'no shutdown', 'ip address 10.0.0.1 255.255.255.255']
Python will unpack info list itself and pass list elements to function as arguments.
Note
Tuple can also be unpacked in this way.
Unpacking keyword alrguments#
Similarly, you can unpack dictionary to pass it as keyword arguments.
Check_passwd function (func_check_pass_optional_param_2.py file):
In [19]: def check_passwd(username, password, min_length=8, check_username=True):
...: if len(password) < min_length:
...: print('Password is too short')
...: return False
...: elif check_username and username in password:
...: print('Password contains username')
...: return False
...: else:
...: print(f'Password for user {username} has passed all checks')
...: return True
...:
List of dictionaries username_passwd where username and password are specified:
In [20]: username_passwd = [{'username': 'cisco', 'password': 'cisco'},
...: {'username': 'nata', 'password': 'natapass'},
...: {'username': 'user', 'password': '123456789'}]
If you pass dictionary to check_passwd function, there is an error:
In [21]: for data in username_passwd:
...: check_passwd(data)
...:
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-21-ad848f85c77f> in <module>
1 for data in username_passwd:
----> 2 check_passwd(data)
3
TypeError: check_passwd() missing 1 required positional argument: 'password'
Error is because the function has taken dictionary as one argument and believes that it lacks only password argument.
If you add ** before passing a dictionary to function, function will work properly:
In [22]: for data in username_passwd:
...: check_passwd(**data)
...:
Password is too short
Password contains username
Password for user user has passed all checks
In [23]: for data in username_passwd:
...: print(data)
...: check_passwd(**data)
...:
{'username': 'cisco', 'password': 'cisco'}
Password is too short
{'username': 'nata', 'password': 'natapass'}
Password contains username
{'username': 'user', 'password': '123456789'}
Password for user user has passed all checks
Python unpacks dictionary and passes it to function as keyword arguments.
The check_passwd(**data) is converted to a check_passwd(username='cisco', password='cisco').