Author: John Au-Yeung

Sometimes, we want to start a function at a given time with Python.

In this article, we’ll look at how to start a function at a given time with Python.

How to start a function at a given time with Python?

To start a function at a given time with Python, we can use the timedelta function to set the delay before running the function.

For instance, we write:

from datetime import datetime, timedelta
import threading


def update():
    print('hello world')


now = datetime.now()
run_at = now + timedelta(seconds=3)
delay = (run_at - now).total_seconds()
threading.Timer(delay, update).start()

to create the update function that we want to run after a 3 second delay.

To do this, we get the current date time with datetime.now.

And then we add the 3 seconds time difference by calling the timedelta function and add the returned time delta object to now.

Next, we calculate the delay with (run_at - now).total_seconds().

Finally, we call the Timer constructor with the delay and the update function to create the thread.

And we call start on the Timer instance to run the function after the specified delay.

Therefore, we should see 'hello world' printed after a 3 seconds delay.

Conclusion

To start a function at a given time with Python, we can use the timedelta function to set the delay before running the function.

Sometimes, we want to split a column of tuples in a Python Pandas data frame.

In this article, we’ll look at how to split a column of tuples in a Python Pandas data frame.

How to split a column of tuples in a Python Pandas data frame?

To split a column of tuples in a Python Pandas data frame, we can use the column’s tolist method.

For instance, we write:

import pandas as pd

df = pd.DataFrame({'a': [1, 2], 'b': [(1, 2), (3, 4)]})
df2 = pd.DataFrame(df['b'].tolist(), index=df.index)
print(df2)

We create the df data frame with the pd.DataFrame class and a dictionary.

Then we create a new data frame from df by using df['b'].tolist() to get column b and convert it to a list.

And we set the index to df.index.

This will split the tuple elements into separate entries in each row.

Therefore, df2 is:

   0  1
0  1  2
1  3  4

Conclusion

To split a column of tuples in a Python Pandas data frame, we can use the column’s tolist method.

Sometimes, we want to convert a list of lists into a Python NumPy array.

In this article, we’ll look at how to convert a list of lists into a Python NumPy array.

How to convert a list of lists into a Python NumPy array?

To convert a list of lists into a Python NumPy array, we can create an array of arrays with the numpy.array method.

For instance, we write:

import numpy

x = [[1, 2], [1, 2, 3], [1]]
y = numpy.array([numpy.array(xi) for xi in x], dtype=object)
print(y)

We call numpy.array with a list that converts the lists inside x to array with numpy.array(xi) for xi in x.

And we set dtype to object.

As a result, we see that y is [array([1, 2]) array([1, 2, 3]) array([1])].

Conclusion

To convert a list of lists into a Python NumPy array, we can create an array of arrays with the numpy.array method.

Sometimes, we want to do list subtraction with Python

In this article, we’ll look at how to do list subtraction with Python

How to do list subtraction with Python??

To do list subtraction with Python, we can convert the 2 lists to sets and then do subtract with them.

And then we can convert the difference back to a list.

For instance, we write:

x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 0]
y = [1, 3, 5, 7, 9]
z = list(set(x) - set(y))
print(z)

We convert x and y to sets with the set function.

Then we get the set difference with set(x) - set(y).

Finally, we convert the set difference set back to a list with list.

Therefore, z is [0, 2, 4, 6, 8].

Conclusion

To do list subtraction with Python, we can convert the 2 lists to sets and then do subtract with them.

And then we can convert the difference back to a list.

Sometimes, we want to find the nth occurrence of substring in a string with Python.

In this article, we’ll look at how to find the nth occurrence of substring in a string with Python.

How to find the nth occurrence of substring in a string with Python?

To find the nth occurrence of substring in a string with Python, we can use a loop to find it.

For instance, we write:

def find_nth(haystack, needle, n):
    start = haystack.find(needle)
    while start >= 0 and n > 1:
        start = haystack.find(needle, start + len(needle))
        n -= 1
    return start


index = find_nth("foofoofoofoo", "foofoo", 2)
print(index)

to define the find_nth function that searches the haystack string for the needle.

And we want to find the nth occurrence of it.

In the function, we call haystack.find with needle to find the index of the first occurrence of needle.

If start is bigger than or equal to 0 and n is bigger than 1, then we start the while loop to use haystack.find(needle, start + len(needle)) to update start until n becomes 0.

Finally, we return start which is the first index of the nth occurrence of the needle.

Therefore, index is 6.

Conclusion

To find the nth occurrence of substring in a string with Python, we can use a loop to find it.