Change Item Value
To change the value of a specific item, refer to the index number:
Example
Change the second item:
thislist = ["apple", "banana", "cherry"]
thislist[1] = "blackcurrant"
print[thislist]
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Change a Range of Item Values
To change the value of items within a specific range, define a list with the new values, and refer to the range of index numbers where you want to insert the new values:
Example
Change the values "banana" and "cherry" with the values "blackcurrant" and "watermelon":
thislist = ["apple", "banana", "cherry", "orange", "kiwi", "mango"]
thislist[1:3] = ["blackcurrant", "watermelon"]
print[thislist]
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If you insert more items than you replace, the new items will be inserted where you specified, and the remaining items will move accordingly:
Example
Change the second value by replacing it with two new values:
thislist = ["apple", "banana", "cherry"]
thislist[1:2] = ["blackcurrant", "watermelon"]
print[thislist]
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Note: The length of the list will change when the number of items inserted does not match the number of items replaced.
If you insert less items than you replace, the new items will be inserted where you specified, and the remaining items will move accordingly:
Example
Change the second and third value by replacing it with one value:
thislist = ["apple", "banana", "cherry"]
thislist[1:3] = ["watermelon"]
print[thislist]
Try it Yourself »
Insert Items
To insert a new list item, without replacing any of the existing values, we can use the insert[] method.
The insert[] method inserts an item at the specified index:
Example
Insert "watermelon" as the third item:
thislist = ["apple", "banana", "cherry"]
thislist.insert[2, "watermelon"]
print[thislist]
Try it Yourself »
Note: As a result of the example above, the list will now contain 4 items.
money + 2 is a no-op. You actually have to assign money to a new value
money = money + 2
# or
money += 2
But then you'll find you get an error - you can't assign to variables outside a function scope. You can use the global keyword:
global money
money += 2
This will allow you to change the value of money within the function.
However, the recommended way is passing money as a parameter:
def gainM[money]:
money += 2
Stats[]
return money
if money == 1:
money = gainM[money]
If you're using the second option [which you should be], you also need to change your
Stats function, to have a money parameter as well.
def Stats[money]:
print
print "money " + str[money]
Otherwise the function will print 1 instead of 3.
Another recommendation - use string formatting.
'money %d' % money # the old way
'money {}'.format[money] # the new and recommended way
Now you pass money into the Stats function.
def gainM[money]:
money += 2
Stats[money]
return money
Hi Everyone, in this article we are going to talk about how can you modify the original behavior of an existing class method or function in Python.
Why would you need it?
Sometimes you call a function very often with arguments having the same value. However, sometimes you may forget and miss an invocation without specifying that value. Using
partial, you can modify the original behavior of that function and make it act the way you want by default, without having to specify that each time.
How do you use it?
The usage of partial is pretty straightforward, let’s give a quick example on it.
Let’s say you always use pandas.read_csv to read a tsv [tab-separated values] file and you don’t want an exception to be raised if a bad line is parsed. You can do as follows:
from functools import partial
import pandas as pdpd.read_csv = partial[pd.read_csv, sep='\t', error_bad_lines=False]
Each call for pd.read_csv after this is going to default to be using tabs as separators instead of commas. But, what if the original behavior is still needed? We can still do better and preserve the original functionality by doing the following:
pd.read_tsv = partial[pd.read_csv, sep='\t', error_bad_lines=False]Now, we have our own version of read_csv that deals only with tsv files. So, this gave us a one-line implementation without having to change the original function.
How does it work?
In
Python, everything works. partial is just a function that takes a function and several keyword-arguments and returns a modified version of that function. Let’s try to implement it from scratch [this is just a simple educational version]:
def my_partial[func, *args, **kwargs]:
def new_funct[*new_args, **new_kwargs]:
new_kwargs.update[kwargs.copy[]]
return func[*args, *new_args, **kwargs]
return new_functWorks with functions! How about methods?
For methods, we always define them in a similar way to this.
class Example:
def my_method[self, arg1]:
...If you use partial to patch this, the self argument will be messed up. Partial would
redefine the method to a function, which leads to self being just a normal argument.
Instead for methods, you will have to use partialmethod from functools. It allows you to patch a method definition without changing them to functions. The usage, in this case, does not differ from partial.
pd.DataFrame.to_tsv = partialmethod[pd.DataFrame.to_csv, sep='\t']Now, whenever we have a dataframe [df for example] will be able to call df.to_tsv right away.
Important Notes:
- Make sure to check the scope of your changes, and make sure any partial change is not going to affect your whole project if you are not careful with your imports.
- You can store your original function in a variable in order to go back to it when needed.
- Make sure you don’t set default values for things like filenames or values to be cast [e.g.
partial[int, x=10]] as in some cases you may unintentionally store large objects in memory without realizing it. - Let it be
known to your teammates that you are using
partialsomewhere in the codebase, otherwise, they would be coding with different assumptions in mind.
Further Reading:
- //docs.python.org/3.8/library/functools.html#functools.partial
- //en.wikipedia.org/wiki/Monkey_patch