Dask is a python library that provides a list of APIs for performing the computation in parallel on a single computer as well as a cluster of computers. We have already created tutorial on dask.bag API which is spark like API for parallelizing big data. Dask has other APIs like
dask.distributed whose introduction we have covered in the previous tutorial. We suggest that you go through that tutorial to get basic information about the dask. We'll be specifically concentrating on
dask.delayed API as a part of this tutorial.
dask.delayed provides a very flexible API which lets us parallelize our python functions. It's very suitable for problems when it doesn't involve data structures like
dask.dataframe. The API of dask.delayed is simplified which lets us parallelize our python code very easily.
dask.delayed API is used to convert normal function to lazy function. When a function is converted from normal to lazy, it prevents function to execute immediately. Instead, its execution is delayed in the future. Dask can easily run these lazy functions in parallel.
dask.delayed API keeps on creating a directed acyclic graph of these lazy functions in the background. This graph keeps track of the execution sequence of these lazy functions which helps it understand which parts can be executed in parallel. When we evaluate this graph by calling
compute() method of dask.delayed API then it evaluates all functions by running them in parallel. We'll be visualizing graphs created by dask when explaining the usage of
This ends our small introduction of
dask.delayed API. We'll be explaining its usage with few examples below.
So without further delay, let’s get started with the coding part.
We'll first import all the necessary libraries for a tutorial.
import dask from dask import delayed, compute import numpy as np import pandas as pd import time import sys import os print("Python Version : ", sys.version) print("Dask Version : ", dask.__version__)
Python Version : 3.7.3 (default, Mar 27 2019, 22:11:17) [GCC 7.3.0] Dask Version : 2.15.0
We can easily parallelize our normal python code using dask by following list of below steps:
dask.delayed. All functions calling these lazy functions will also become lazy.
Scenario 1:You only need to execute one final function which calls a list of lazy functions. If this is the scenario then you can call compute() on final lazy function and it'll run all lazy functions it depends on in parallel.
Scenario 2:If there are more than one final lazy functions then we can use
dask. We need to pass all lazy functions to this method and it'll run all of them in parallel and returns results for all of them.
We'll be explaining these steps below with few examples to make things clear. We'll be creating normal python functions which takes time to execute normally but runs faster when parallelized using dask.
Dask provides an optional dashboard which we can utilize to see the performance of operations getting performed in parallel. This step is optional if you don't want to analyze results but it can be very useful to debug things.
from dask.distributed import Client client = Client(n_workers=4, threads_per_worker=4) client
We suggest that you pass
n_workers same as a number of cores on your PC and
threads_per_worker as 1 if you are running code on local.
You can open the URL mentioned next to the dashboard. It'll have a list of tabs that provides information like running time of the task, CPU usage, memory usage, directed graphs, etc. All this can be useful for analyzing parallel execution of tasks. You'll be able to see the whole directed acyclic graph of lazy functions as well in the
Graph section of the dashboard. They will be all marked
blue once that particular lazy function node has completed execution. If your function has many lazy functions then you'll be able to see changing their values in real-time to
blue when they complete.
Below we have created a simple python example which takes the square of two numbers and adds them. We have created a function for getting square of numbers. We have artificially added sleep time of 1 second to function to mimic a real-life situations where functions can take time to execute. We are calling this function 2 times to get a square of 2 different numbers before adding them. As per below pure python code, it executes
square function sequentially which results in taking 2 seconds for code to complete.
Please make a note that we'll be using %%time and %time jupyter notebook magic commands to measure execution of particular code section. It'll be used for performance comparison between normal python and dask parallelized python code.
%%time def square(x): time.sleep(1) return x*x x_sq = square(3) y_sq = square(4) res = x_sq + y_sq res
CPU times: user 149 ms, sys: 14.4 ms, total: 163 ms Wall time: 2 s
We'll now try to convert the above example to the dask version so that it can run fast using parallelization. From the above code, we can easily notice that
square function takes time to execute. We can even notice that each execution of the square is totally independent of any other execution of the same function which makes it ideal for parallel execution.
We'll convert each square function execution to
lazy function by calling
delayed on them as explained below.
%%time x_sq = delayed(square)(3) y_sq = delayed(square)(4) res = x_sq + y_sq res
CPU times: user 2.89 ms, sys: 0 ns, total: 2.89 ms Wall time: 2.66 ms
We can notice from above cell execution that it executed immediately. It's because of dask.delayed API has just converted normal square execution to lazy execution and has created an execution graph as of now. But it has not actually executed functions. We can see that the return type of res is
Delayed object instead of the result.
Please make a note on process of creating lazy function in above example. We need to pass original function name to delayed() method and then list of parameters in another parenthesis. Please feel free to go through this list of dask best practices which will prevent you from making common mistakes.
We can make dask execute code in parallel by calling
compute() method on the final
res object. It'll evaluate the graph and execute a square function in parallel.
CPU times: user 110 ms, sys: 15.8 ms, total: 125 ms Wall time: 1.04 s
We can notice from the above cell that it takes less time to compute the sum of squares using
We can even visualize DAG created by dask by calling
visualize() method on the final
Below we are explaining another usage of dask.delayed API. Here we are creating an array of random numbers of size 10x1000. We then want to compute the correlation between the first row of the array and all other rows. We have implemented this solution as a loop. We also have designed the function
corr which calculates correlation. We have added a time delay of 1 second into function execution to mimic real-life situations.
%%time x = np.random.rand(10,1000) def corr(x, y): time.sleep(1) return np.corrcoef(x, y) correlations =  for row in x: correlations.append(corr(x, row)) avg_corr = np.mean(correlations) avg_corr
CPU times: user 922 ms, sys: 117 ms, total: 1.04 s Wall time: 10 s
We can notice that the above code takes 10 seconds for execution because there are 10 rows in an array. We are computing a correlation between the first row and all other rows, all of which take 1 second.
We are below converting the above code to dask delayed code so that it runs in parallel. We have used delayed at two places this time. First on
corr() function and then on
%%time def corr(x, y): time.sleep(1) return np.corrcoef(x, y) correlations =  for row in x: correlations.append(delayed(corr)(x, row)) avg_corr = delayed(np.mean)(correlations) avg_corr
CPU times: user 3.13 ms, sys: 0 ns, total: 3.13 ms Wall time: 3.13 ms
Like the previous example, we can notice that the above code with lazy functions executes fast because actual execution of code has not happened yet.
We'll not call
compute() on final
avg_corr variable to complete execution.
CPU times: user 146 ms, sys: 14.7 ms, total: 161 ms Wall time: 1.23 s
We can notice that it completes execution is quite less time compared to normal sequential execution.
Below we are visualizing the execution graph of the above code.
Below we are demonstrating usage of
dask.delayed API using the third example. We are first looping through number 1-10 taking the square of the number if the number is divisible by 2 else taking power 3 of number. We have defined function for taking a square and power 3 of numbers. We have also added time delay in function so that it looks like a real-life situation.
%%time def square(x): time.sleep(1) return x*x def power_3(x): time.sleep(1) return x*x*x final_list =  for i in range(1,11): if i%2 == 0: final_list.append(square(i)) else: final_list.append(power_3(i)) res = sum(final_list) res
CPU times: user 896 ms, sys: 131 ms, total: 1.03 s Wall time: 10 s
We can see above that when we run above loop in parallel then it takes 10 seconds for it to complete. We can easily convert this code using
dask.delayed as explained below. This example demonstrates how conditional functions can also be parallelized using dask.
We have made use of
@delayed annotation this time instead of wrapping calls of function like previous examples. We can also use the
annotation above function rather than wrapping each call of function into
%%time @delayed def square(x): time.sleep(1) return x*x @delayed def power_3(x): time.sleep(1) return x*x*x final_list =  for i in range(1,11): if i%2 == 0: final_list.append(square(i)) else: final_list.append(power_3(i)) res = delayed(sum)(final_list) res
CPU times: user 3.09 ms, sys: 3.31 ms, total: 6.39 ms Wall time: 4.22 ms
CPU times: user 134 ms, sys: 16.4 ms, total: 150 ms Wall time: 1.06 s
As a part of our 4th example, we'll be working with some files. We are creating 30 CSV files for 30 days of temperature data where the temperature is recorded every 5 seconds. We are first creating CSV files into a separate folder for each day. Each CSV file will have 17280 temperature values for that day recorded at every 5 seconds. We are filling temperature data with a random temperature between 1-50.
if not os.path.exists("temp_dataframes"): os.mkdir("temp_dataframes") for i in range(1,31): dates= pd.date_range(start="1-%d-2020"%i, end="1-%d-2020"%(i+1), freq="5S")[:-1] temp_df = pd.DataFrame(np.random.randint(1,50, 17280), index=dates, columns=["Temp"]) temp_df.to_csv("temp_dataframes/1-%d-2020.csv"%i)
Below we are looping through temperature file of each day, reading it, taking an average of that day, and adding it to list. We are then taking an average of all days combined. Our main aim of this example was to explain the usage of
dask.delayed API working with files.
%%time def calc_mean(f_name): df = pd.read_csv(f_name) return df.mean().values avg_temp_per_day =  for i in range(1,31): avg_temp_per_day.append(calc_mean("temp_dataframes/1-%d-2020.csv"%i)) avg_temp = np.mean(avg_temp_per_day) avg_temp
CPU times: user 11.7 s, sys: 136 ms, total: 11.9 s Wall time: 11.7 s
We can see that it takes nearly 12 seconds for the above code to complete.
We have converted it to dask code below which completes in nearly 3 seconds.
%%time @delayed def calc_mean(f_name): df = pd.read_csv(f_name) return df.mean().values avg_temp_per_day =  for i in range(1,31): avg_temp_per_day.append(calc_mean("temp_dataframes/1-%d-2020.csv"%i)) avg_temp = delayed(np.mean)(avg_temp_per_day) avg_temp
CPU times: user 0 ns, sys: 2.88 ms, total: 2.88 ms Wall time: 2.23 ms
CPU times: user 141 ms, sys: 29.1 ms, total: 170 ms Wall time: 3.95 s
Our fifth and last example is designed to explain the usage of
compute() method available directly from dask which takes an input list of lazy objects and can evaluate them.
Below we are looping through 1-10, creating an array of random numbers of size 100 between 1-100. We are then taking sum and count of numbers divisible by 2. We add all sums and counts to an array. We then sum the array of sums and divide it by sum of counts array to get average. We have added time delay to function so that it takes time to mimicking real-life situations.
%%time def total_func(x): time.sleep(0.5) return sum([i for i in x if i%2==0]) def count_func(x): time.sleep(0.5) return len([i for i in x if i%2==0]) totals, counts = ,  for i in range(10): x = np.random.randint(1,100,1000) total = total_func(x) count = count_func(x) totals.append(total) counts.append(count) avg_num = sum(totals) / sum(counts) avg_num
CPU times: user 963 ms, sys: 180 ms, total: 1.14 s Wall time: 10.1 s
Below we have converted the above example to
dask.delayed version. We have annotated both functions with
@delayed annotation. We have evaluated both
counts_orig array which are list of delayed objects using
compute() function. We can see that this one takes nearly 1 second to complete compared to the previous version.
%%time @delayed def total_func(x): time.sleep(0.5) return sum([i for i in x if i%2==0]) @delayed def count_func(x): time.sleep(0.5) return len([i for i in x if i%2==0]) totals_orig, counts_orig = ,  for i in range(10): x = np.random.randint(1,100,1000) total = total_func(x) count = count_func(x) totals_orig.append(total) counts_orig.append(count) totals, counts = compute(totals_orig, counts_orig) avg_num = sum(totals) / sum(counts) avg_num
CPU times: user 168 ms, sys: 28.8 ms, total: 196 ms Wall time: 1.09 s
[Delayed('total_func-85fc3673-c004-45d1-81f8-f08509e7d79a'), Delayed('total_func-10b5ebca-d6f5-45f7-b2d1-882c7391fd7e'), Delayed('total_func-6c44bd15-fc20-4682-82cb-6446a414eab7'), Delayed('total_func-e50378bb-c06c-414e-b57e-71d123aff579'), Delayed('total_func-6ce847de-c7b4-4b08-9207-751a6733b8ee'), Delayed('total_func-30e0b292-ad4c-4ae9-b8f4-bb615171eaee'), Delayed('total_func-24609ca7-50b8-40fd-ad94-fad5214a5338'), Delayed('total_func-b512e024-9c64-426b-8036-09ba355daa85'), Delayed('total_func-6dd12b7c-9a4c-44b5-8a52-a803d44432f3'), Delayed('total_func-b6ac83a0-c108-4d20-8c68-5ed3ca982d2e')]
[Delayed('count_func-42da9b82-ca2f-4294-8b0e-5013bd30f3d9'), Delayed('count_func-c8124297-4dea-432f-8e0a-e709bc8294a2'), Delayed('count_func-de8a79e4-85c7-4d39-942c-47890c647b6b'), Delayed('count_func-6d64ef92-d561-42ca-a507-902d01906e99'), Delayed('count_func-afde8147-4294-4d7b-af24-829933747b0d'), Delayed('count_func-85a4b50d-b988-4e3c-ad35-0779124ded27'), Delayed('count_func-03b89eae-bc9c-4456-9f58-3643c53bfbd8'), Delayed('count_func-0155b304-7f18-4016-93af-4cbf9e84a867'), Delayed('count_func-b8ab072d-e1e0-4846-add2-5b99e8789cd4'), Delayed('count_func-951b32a7-7860-49f1-af7c-258a8ce1aaf7')]
This ends our small tutorial explaining the usage of dask.delayed API for parallelizing python code. Please feel free to let us know your views in the comments section.
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