Category: Programming

Problem Formulation

» Problem Statement: Given a NumPy array. How to print the NumPy array without scientific notation in Python?

Note: Python represents very small or very huge floating-point numbers in their scientific form. Scientific notation represents the number in terms of powers of 10 to display very large or very small numbers. For example, the scientific notation for the number 0.000000321 is described as 3.21e07. 

In Python, the NumPy module generally uses scientific notation instead of the actual number while printing/displaying the array items.

Example: Look at the following code snippet:

arr = np.array([1, 5, 10, 20, 35, 5000.5])
print(arr)

Output:

[1.0000e+00 5.0000e+00 1.0000e+01 2.0000e+01 3.5000e+01 5.0005e+03]

Expected Output: Print the given array without scientific notation in Python as:

[ 1. 5. 10. 20. 35. 5000.5]

Without further ado, let’s dive into the different ways of solving the given problem.

☛Method 1: Using set_printoptions() Function

The set_printoptions() is a function in the numpy module that is used to set how the floating-point numbers, NumPy arrays and numpy objects are to be displayed. By default, the very big or very small numbers of the array are represented using scientific notation. We can use the set_printoptions() function by passing the suppress as True to remove the scientific notation of the numpy array.

Approach:

• Import the Numpy module to create the array.
• Use the set_printoptions() function and pass the suppress value as True.
• Print the array; it will get displayed without the scientific notation.

Code:

# Importing the numpy module
import numpy as np
# Creating a NumPy array
a = np.array([1, 5, 10, 20, 35, 5000.5])
print("Numpy array with scientific notation", a)
np.set_printoptions(suppress = True)
print("Numpy array without scientific notation", a)

Output:

Numpy array with scientific notation [1.0000e+00 5.0000e+00 1.0000e+01 2.0000e+01 3.5000e+01 5.0005e+03]
Numpy array without scientific notation [ 1. 5. 10. 20. 35. 5000.5]

Discussion: The set_printoptions() function only works for the numbers that fit in the default 8-character space allotted to it, as shown below:

Code:

import numpy as np
# Array with element index 1 having 8 digits
a = np.array([5.05e-5, 15.6, 2.1445678e5])
print("Numpy array with scientific notation", a)
np.set_printoptions(suppress = True)
print("Numpy array without scientific notation", a)

Output:

Numpy array with scientific notation [5.0500000e-05 1.5600000e+01 2.1445678e+05]
Numpy array without scientific notation [ 0.0000505 15.6 214456.78 ]

When we pass a number that is greater than 8 characters wide, exponential notation is imposed as shown below:

Code:

import numpy as np
# Array with element index 1 having more than 8 digits
a = np.array([5.05e-5, 15.6, 2.1445678e10])
print("Numpy array with scientific notation", a)
np.set_printoptions(suppress = True)
print("Numpy array without scientific notation", a)

Output:

Numpy array with scientific notation [5.0500000e05 1.5600000e+01 2.1445678e+10]
Numpy array without scientific notation [5.0500000e05 1.5600000e+01 2.1445678e+10]

☛Method 2: Using set_printoptions() Function with .format

As in method 1, the set_printoptions() function does not work when the number has more than eight characters. That is when set_printoptions(formatter) is used to specify the options for printing and rounding. We have to set the function to print the float variable.

Python’s built-in format(value, spec) function transforms the input of one format into the output of another format defined by you. Specifically, it applies the format specifier spec to the argument value and returns a formatted representation of value. Read more about the “Python format() Function.”

Code:

import numpy as np
# Creating a NumPy array
# Array with element index 1 having more than 8 digits
a = np.array([5.05e-5, 15.6, 2.1445678e10])
print("Numpy array with scientific notation", a)
np.set_printoptions(suppress = True, formatter = {'float_kind':'{:f}'.format})
print("Numpy array without scientific notation", a)

Output:

Numpy array with scientific notation [5.0500000e-05 1.5600000e+01 2.1445678e+10]
Numpy array without scientific notation [0.000051 15.600000 21445678000.000000]

We can also format the output to only have 2 units precision by using '{:0.2f}' .format as shown below:

Code:

import numpy as np
# Array with element index 1 having more than 8 digits
a = np.array([5.05e-5, 15.6, 2.1445678e10])
print("Numpy array with scientific notation", a)
np.set_printoptions(suppress = True, formatter = {'float_kind':'{:0.2f}'.format})
print("Numpy array without scientific notation", a)

Output:

Numpy array with scientific notation [5.0500000e-05 1.5600000e+01 2.1445678e+10]
Numpy array without scientific notation [0.00 15.60 21445678000.00]

Discussion: The disadvantage of using this method to suppress the exponential notion in the numpy arrays is when the array gets a very large float value. When we try to print this array, we are going to get a whole page of numbers.

☛Method 3: Using printoptions() Function

The printoption() function is a function in the Numpy module used as a context manager for setting print options. By passing the precision as 3 and suppress as True in the printoptions() function, we can remove the scientific notation and print the Numpy array.

Note: This function only works if you use NumPy versions 1.15.0 or later.

Approach:

• Import the numpy module to create the array.
• Use the printoption() function inside the “with” and pass the precision value as 3 and the suppress value as True.
• Print the array; it will get displayed without the scientific notation.

Code:

import numpy as np
# Creating a NumPy array
a = np.array([1, 5, 10, 20, 35, 5000.5])
print("Numpy array with scientific notation", a)
print("Numpy array without scientific notation:")
with np.printoptions(precision = 3, suppress = True): print(a)

Output:

Numpy array with scientific notation [1.0000e+00 5.0000e+00 1.0000e+01 2.0000e+01 3.5000e+01 5.0005e+03]
Numpy array without scientific notation: [ 1. 5. 10. 20. 35. 5000.5]

☛Method 4: Using array2string() Function

The array2string() is a function in the numpy module that returns a string representation of an array. We can use this function to print a NumPy array without scientific notation by passing the array as the argument and setting the suppress_small argument as True. When the suppress_small argument is True, it represents the numbers close to zero as zero.

Approach:

• Import the numpy module to create the array.
• Use the array2string() function and pass the suppress_small argument as True.
• Finally, print the array. It will get displayed without the scientific notation.

Code:

import numpy as np
# Creating a NumPy array
a = np.array([1, 5, 10, 20, 35, 5000.5])
print("Numpy array with scientific notation", a)
a = np.array2string(a, suppress_small = True)
print("Numpy array without scientific notation:", a)

Output:

Numpy array with scientific notation [1.0000e+00 5.0000e+00 1.0000e+01 2.0000e+01 3.5000e+01 5.0005e+03]
Numpy array without scientific notation: [ 1. 5. 10. 20. 35. 5000.5]

Conclusion

Hurrah! We have successfully solved the mission-critical question in numerous ways in this article. I hope you found it helpful. Please stay tuned and subscribe for more such interesting articles. 

Interesting Read: How to Suppress Scientific Notation in Python?

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Problem Statement: Consider that you have been given a list in Python. How will you count the number of unique values in the list?

Example: Let’s visualize the problem with the help of an example:

Given:  li = [‘a’, ‘a’, ‘b’, ‘c’, ‘b’, ‘d’, ‘d’, ‘a’] Output: The unique values in the given list are ‘a’, ‘b’, ‘c’, ‘d’. Thus the expected output is 4.

Now that you have a clear picture of what the question demands, let’s dive into the different ways of solving the problem.

Method 1: The Naive Approach

Approach:

• Create an empty list that will be used to store all the unique elements from the given list. Let’s say that the name of this list res.
• To store the unique elements in the new list that you created previously, simply traverse through all the elements of the given list with the help of a for loop and then check if each value from the given list is present in the list “res“.
  • If a particular value from the given list is not present in the newly created list then append it to the list res. This ensures that each unique value/item from the given list gets stored within res.
  • If it’s already present, then do not append the value.
• Finally, the list res represents a newly formed list that contains all unique values from the originally given list. All that remains to be done is to find the length of the list res which gives you the number of unique values present in the given list.

Code:

# Given list
li = ['a', 'a', 'b', 'c', 'b', 'd', 'd', 'a']
res = []
for ele in li: if ele not in res: res.append(ele)
print("The count of unique values in the list:", len(res)) # The count of unique values in the list: 4

Discussion: Since you have to create an extra list to store the unique values, this approach is not the most efficient way to find and count the unique values in a list as it takes a lot of time and space.

Method 2: Using set()

A more effective and pythonic approach to solve the given problem is to use the set() method. Set is a built-in data type that does not contain any duplicate elements.

Read more about sets here – “The Ultimate Guide to Python Sets“

Approach: Convert the given list into a set using the set() function. Since a set cannot contain duplicate values, only the unique values from the list will be stored within the set. Now that you have all the unique values at your disposal, you can simply count the number of unique values with the help of the len() function.

Code:

li = ['a', 'a', 'b', 'c', 'b', 'd', 'd', 'a']
s = set(li)
unique_values = len(s)
print("The count of unique values in the list:", unique_values) # The count of unique values in the list: 4

You can formulate the above solution in a single line of code by simply chaining both the functions (set() and len()) together, as shown below:

# Given list
li = ['a', 'a', 'b', 'c', 'b', 'd', 'd', 'a']
# One-liner
print("The count of unique values in the list:", len(set(li)))

Method 3: Using Dictionary fromkeys()

Python dictionaries have a method known as fromkeys() that is used to return a new dictionary from the given iterable ( such as list, set, string, tuple) as keys and with the specified value. If the value is not specified by default, it will be considered as None. 

Approach: Well! We all know that keys in a dictionary must be unique. Thus, we will pass the list to the fromkeys() method and then use only the key values of this dictionary to get the unique values from the list. Once we have stored all the unique values of the given list stored into another list, all that remains to be done is to find the length of the list containing the unique values which will return us the number of unique values.

Code:

# Given list
li = ['a', 'a', 'b', 'c', 'b', 'd', 'd', 'a']
# Using dictionary fromkeys()
# list elements get converted to dictionary keys. Keys are always unique!
x = dict.fromkeys(li)
# storing the keys of the dictionary in a list
l2 = list(x.keys())
print("Number of unique values in the list:", len(l2)) # Number of unique values in the list: 4

Method 4: Using Counter

Another way to solve the given problem is to use the Counter function from the collections module. The Counter function creates a dictionary where the dictionary’s keys represent the unique items of the list, and the corresponding values represent the count of a key (i.e. the number of occurrences of an item in the list). Once you have the dictionary all you need to do is to extract the keys of the dictionary and store them in a list and then find the length of this list.

from collections import Counter
# Given list
li = ['a', 'a', 'b', 'c', 'b', 'd', 'd', 'a']
# Creating a list containing the keys (the unique values)
key = Counter(li).keys()
# Calculating the length to get the count
res = len(key)
print("The count of unique values in the list:", res) # The count of unique values in the list: 4

Method 5: Using Numpy Module

We can also use Python’s Numpy module to get the count of unique values from the list. First, we must import the NumPy module into the code to use the numpy.unique() function that returns the unique values from the list.

Solution:

# Importing the numpy module
import numpy as np
# Given list
li = ['a', 'a', 'b', 'c', 'b', 'd', 'd', 'a']
res = []
# Using unique() function from numpy module
for ele in np.unique(li): res.append(ele)
# Calculating the length to get the count of unique elements
count = len(res)
print("The count of unique values in the list:", count) # The count of unique values in the list: 4

Another approach is to create an array using the array() function after importing the numpy module. Further, we will use the unique() function to remove the duplicate elements from the list. Finally, we will calculate the length of that array to get the count of the unique elements.

Solution:

# Importing the numpy module
import numpy as np
# Given list
li = ['a', 'a', 'b', 'c', 'b', 'd', 'd', 'a']
array = np.array(li)
u = np.unique(array)
c = len(u)
print("The count of unique values in the list:", c) # The count of unique values in the list: 4

Method 6: Using List Comprehension

There’s yet another way of solving the given problem. You can use a list comprehension to get the count of each element in the list and then use the zip() function to create a zip object that creates pairs of each item along with the count of each item in the list. Store these paired items as key-value pairs in a dictionary by converting the zip object to a dictionary using the dict() function. Finally, return the dictionary’s keys’ calculated length (using the len() function).

Code:

# Given list
li = ['a', 'a', 'b', 'c', 'b', 'd', 'd', 'a']
# List comprehension using zip()
l2 = dict(zip(li, [li.count(i) for i in li]))
# Using len to get the count of unique elements
l = len(list(l2.keys()))
print("The count of the unique values in the list:", l) # The count of the unique values in the list: 4

Conclusion

In this article, we learned the different methods to count the unique values in a list in Python. We looked at how to do this using the counter, sets, numpy module, and list comprehensions. If you found this article helpful and want to receive more interesting solutions and discussions in the future, please subscribe and stay tuned!

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Welcome to the bonus content of “The Book of Dash”.

Here you will find additional examples of Plotly Dash components, layouts and style. To learn more about making dashboards with Plotly Dash, and how to buy your copy of “The Book of Dash”, please see the reference section at the bottom of this article.

As you read the article, feel free to run the explainer video on the Card components from one of our coauthors’ “Charming Data” YT channel:

This article will focus on the Card components from the Dash Boostrap Component library. Using cards is a great way to create eye-catching content. We’ll show you how to make the card content interactive with callbacks, but first we’ll focus on the style and layout.

Plotly Dash App with a Bootstrap Card

We’ll start with the basics – a minimal Dash app to display a single card without any additional styling. Be sure to check out the complete reference for using Dash Bootstrap cards.

Next, we’ll show how to jazz it up to make it look better — and more importantly — so it conveys key information at a glance.

from dash import Dash, html
import dash_bootstrap_components as dbc app = Dash(__name__, external_stylesheets=[dbc.themes.SPACELAB, dbc.icons.BOOTSTRAP]) card = dbc.Card( dbc.CardBody( [ html.H1("Sales"), html.H3("$104.2M") ], ),
) app.layout=dbc.Container(card) if __name__ == "__main__": app.run_server(debug=True)

Styling a Dash Bootstrap Card

An easy way to style content is by using Boostrap utility classes. See all the utility classes at the Dash Bootstrap Cheatsheet app. This handy cheatsheet is made by a co-author of “The Book of Dash”.

In this card, we center the text and change the color with “text-center” and “text-success“. The Bootstrap themes have named colors and “success” is a shade of green.

Recommended Resource: For more information about styling your app with a Boostrap theme, see Dash Bootstrap Theme Explorer

card = dbc.Card( dbc.CardBody( [ html.H1("Sales"), html.H3("$104.2M", className="text-success") ], ), className="text-center"
)

Feel free to watch Adam’s explainer video on Bootstrap and styling your app if you need to get up to speed!

Dash Bootstrap Card with Icons

You can add Bootstrap and/or Font Awesome icons to your Dash Bootstrap components. In this example, we will add the bank icon as well as change the background color using the Bootstrap utility class bg-primary.

card = dbc.Card( dbc.CardBody( [ html.H1([html.I(className="bi bi-bank me-2"), "Profit"]), html.H3("$8.3M"), html.H4(html.I("10.3% vs LY", className="bi bi-caret-up-fill text-success")), ], ), className="text-center m-4 bg-primary text-white",
)

To learn more, see the Icons section of the dash-bootstrap-components documentation. You can also find more information about adding icons to dash components in the buttons article.

Recommended Tutorial: Plotly Dash Button Component – A Simple Illustrated Guide

Dash Bootstrap Cards Side-by-Side

In business intelligence dashboards, it’s common to highlight KPIs or Key Performance Indicators in a group of cards. You can find many examples in the Plotly App Gallery:

This app places three KPI cards side-by-side. We use the dbc.Row and dbc.Col components to create this responsive card layout. When you run this app, try changing the width of the browser window to see how the cards expand to fill the row based on the screen size.

This app also demonstrates the usage of Bootstrap border utility classes to add and style a border. Here we add a border on the left and change the color to highlight the results. Another trick is to use the “text-nowrap” class to keep the icon and the text together on the same line when the cards shrink to accommodate small screen sizes.

from dash import Dash, html
import dash_bootstrap_components as dbc app = Dash(__name__, external_stylesheets=[dbc.themes.SPACELAB, dbc.icons.BOOTSTRAP]) card_sales = dbc.Card( dbc.CardBody( [ html.H1([html.I(className="bi bi-currency-dollar me-2"), "Sales"], className="text-nowrap"), html.H3("$106.7M"), html.Div( [html.I("5.8%", className="bi bi-caret-up-fill text-success"), " vs LY",] ), ], className="border-start border-success border-5" ), className="text-center m-4"
) card_profit = dbc.Card( dbc.CardBody( [ html.H1([html.I(className="bi bi-bank me-2"), "Profit"], className="text-nowrap"), html.H3("$8.3M",), html.Div( [ html.I("12.3%", className="bi bi-caret-down-fill text-danger"), " vs LY", ] ), ], className="border-start border-danger border-5" ), className="text-center m-4",
) card_orders = dbc.Card( dbc.CardBody( [ html.H1([html.I(className="bi bi-cart me-2"), "Orders"], className="text-nowrap"), html.H3("91.4K"), html.Div( [ html.I("10.3%", className="bi bi-caret-up-fill text-success"), " vs LY", ] ), ], className="border-start border-success border-5" ), className="text-center m-4",
) app.layout = dbc.Container( dbc.Row( [dbc.Col(card_sales), dbc.Col(card_profit), dbc.Col(card_orders)], ), fluid=True,
) if __name__ == "__main__": app.run_server(debug=True)

Creating Dash Bootstrap Cards in a Loop

In the previous example, notice that a lot of the code for creating the card is the same. To reduce the amount of repetitive code, let’s create cards in a function.

In this app, we introduce the dbc.CardHeader component and the "shadow" class to style the card. We’ll show you how to add more style later in the app that displays crypto prices.

from dash import Dash, html
import dash_bootstrap_components as dbc app = Dash(__name__, external_stylesheets=[dbc.themes.SPACELAB]) summary = {"Sales": "$100K", "Profit": "$5K", "Orders": "6K", "Customers": "300"} def make_card(title, amount): return dbc.Card( [ dbc.CardHeader(html.H2(title)), dbc.CardBody(html.H3(amount, id=title)), ], className="text-center shadow", ) app.layout = dbc.Container( dbc.Row([dbc.Col(make_card(k, v)) for k, v in summary.items()], className="my-4"), fluid=True,
) if __name__ == "__main__": app.run_server(debug=True)

Dash Bootstrap Card with an Image

This card uses the dbc.CardImage component. This is a great format for the “who’s who” section of your app. It works well for displaying information about products too.

from dash import Dash, html
import dash_bootstrap_components as dbc app = Dash(__name__, external_stylesheets=[dbc.themes.SPACELAB]) count = "https://user-images.githubusercontent.com/72614349/194616425-107a62f9-06b3-4b84-ac89-2c42e04c00ac.png" card = dbc.Card([ dbc.CardImg(src=count, top=True), dbc.CardBody( [ html.H3("Count von Count", className="text-primary"), html.Div("Chief Financial Officer"), html.Div("Sesame Street, Inc.", className="small"), ] )], className="shadow my-2", style={"maxWidth": 350},
) app.layout=dbc.Container(card) if __name__ == "__main__": app.run_server(debug=True)

Dash Bootstrap Card with an Image and a Link

This app has a card with the dbc.CardLink component.

When you run this app, try clicking on either the logo or the title. You will see that both are links to the Plotly site displaying the current job openings.

We do this by including both the html.Img component with the Plotly logo and the html.Span with the title in the dbc.CardLink component.

from dash import Dash, html
import dash_bootstrap_components as dbc app = Dash(__name__, external_stylesheets=[dbc.themes.SPACELAB]) plotly_logo_dark = "https://user-images.githubusercontent.com/72614349/182967824-c73218d8-acbf-4aab-b1ad-7eb35669b781.png" card = dbc.Card( dbc.CardBody( [ dbc.CardLink( [ html.Img(src=plotly_logo_dark, height=65), html.Span("Plotly Job Openings", className="ms-2") ], className="text-decoration-none h2", href="https://plotly.com/careers/" ), html.Hr(), html.Div("Engineering", className="h3"), html.Div("Intermediate Backend Engineer", className="text-danger"), html.Div("Remote, Canada", className="small"), ] ), className="shadow my-2", style={"maxWidth": 450},
) app.layout=dbc.Container(card) if __name__ == "__main__": app.run_server(debug=True)

Dash Bootstrap Card with a Background Image

This app puts the image in the background and uses the dbc.CardImgOverlay component to place content on top of the image.

We also use dbc.Buttons to link to other sites for more information. See the buttons article for more information. Be sure to run the app and check out the links. The Webb Telescope app is pretty cool!

Recommended Tutorial: Before After Image in Plotly Dash

from dash import Dash, html
import dash_bootstrap_components as dbc app = Dash(__name__, external_stylesheets=[dbc.themes.SPACELAB, dbc.icons.BOOTSTRAP]) webb_deep_field = "https://user-images.githubusercontent.com/72614349/192781103-2ca62422-2204-41ab-9480-a730fc4e28d7.png"
card = dbc.Card( [ dbc.CardImg(src=webb_deep_field), dbc.CardImgOverlay([ html.H2("James Webb Space Telescope"), html.H3("First Images"), html.P( "Learn how to make an app to compare before and after images of Hubble vs Webb with ~40 lines of Python", style={"marginTop":175}, className="small", ), dbc.Button("See the App", href="https://jwt.pythonanywhere.com/"), dbc.Button( [html.I(className="bi bi-github me-2"), "source code"], className="ms-2 text-white", href="https://github.com/AnnMarieW/webb-compare", ) ]) ], style={"maxWidth": 500}, className="my-4 text-center text-white"
) app.layout=dbc.Container(card) if __name__ == "__main__": app.run_server(debug=True)

See this Plotly Dash app live: https://jwt.pythonanywhere.com/

Plotly Dash App with Live Updates

This app shows live updates of crypto prices. We use a dcc.Interval component to fetch the data from CoinGecko every 6 seconds.

The CoinGecko API is easy to use because you don’t need an API key, and it’s free if you keep the number of updates within the free tier limits. We pull the current price, 24 hour price change, and the coin logo from the data feed and display the data in a nicely styled card.

In this app we introduce callbacks to update the data, and show how to get the data from CoinGecko. All the other styling has been covered in previous examples.

Note that in this app, the color of the text and the up and down arrows are updated dynamically based on the data in the make_card function.

import dash
from dash import Dash, dcc, html, Input, Output
import dash_bootstrap_components as dbc
import requests app = Dash(__name__, external_stylesheets=[dbc.themes.SUPERHERO, dbc.icons.BOOTSTRAP]) coins = ["bitcoin", "ethereum", "binancecoin", "ripple"]
interval = 6000 # update frequency - adjust to keep within free tier
api_url = "https://api.coingecko.com/api/v3/coins/markets?vs_currency=usd" def get_data(): try: response = requests.get(api_url, timeout=1) return response.json() except requests.exceptions.RequestException as e: print(e) def make_card(coin): change = coin["price_change_percentage_24h"] price = coin["current_price"] color = "danger" if change < 0 else "success" icon = "bi bi-arrow-down" if change < 0 else "bi bi-arrow-up" return dbc.Card( html.Div( [ html.H4( [ html.Img(src=coin["image"], height=35, className="me-1"), coin["name"], ] ), html.H4(f"${price:,}"), html.H5( [f"{round(change, 2)}%", html.I(className=icon), " 24hr"], className=f"text-{color}", ), ], className=f"border-{color} border-start border-5", ), className="text-center text-nowrap my-2 p-2", ) mention = html.A( "Data from CoinGecko", href="https://www.coingecko.com/en/api", className="small"
)
interval = dcc.Interval(interval=interval)
cards = html.Div()
app.layout = dbc.Container([interval, cards, mention], className="my-5") @app.callback(Output(cards, "children"), Input(interval, "n_intervals"))
def update_cards(_): coin_data = get_data() if coin_data is None or type(coin_data) is dict: return dash.no_update # make a list of cards with updated prices coin_cards = [] updated = None for coin in coin_data: if coin["id"] in coins: updated = coin.get("last_updated") coin_cards.append(make_card(coin)) # make the card layout card_layout = [ dbc.Row([dbc.Col(card, md=3) for card in coin_cards]), dbc.Row(dbc.Col(f"Last Updated {updated}")), ] return card_layout if __name__ == "__main__": app.run_server(debug=True)

Plotly Dash App with a Sidebar

A common layout for Dash apps is to put inputs in a sidebar, and the output in the main section of the page. We can place both the sidebar and the output in Dash Boostrap Card components.

See the app and the code live at the Dash Example Index

Plotly Dash Example Index

See more examples of interactive apps in the Dash Example Index

Reference

Order Your Copy of “The Book of Dash” Today!

• Dash documentation – tutorial. Getting Started with Dash
• Dash Example Index. Sample apps with sliders
• Dash Bootstrap Components documentation – Themes
• Dash Bootstrap Components documentation – dbc.Card
• Dash Bootstrap Theme Explorer. A guide for styling Plotly Dash apps with a Bootstrap theme

The Book of Dash Authors

Feel free to learn more about the book’s coauthors here:

Ann Marie Ward:

• GitHub: https://github.com/AnnMarieW
• Dash Forum: https://community.plotly.com/u/annmariew/summary

Adam Schroeder:

• YouTube CharmingData: https://www.youtube.com/c/CharmingData

Chris Mayer:

• Python + Crypto Email Academy: https://blog.finxter.com/subscribe/

---

Do you encounter the following error message?

TypeError: 'dict_keys' object is not subscriptable

You’re not alone! This short tutorial will show you why this error occurs, how to fix it, and how to never make the same mistake again.

So, let’s get started!

Solution

Python raises the “TypeError: 'dict_keys' object is not subscriptable” if you use indexing or slicing on the dict_keys object obtained with dict.keys(). To solve the error, convert the dict_keys object to a list such as in list(my_dict.keys())[0].

print(list(my_dict.keys())[0])

Example

The following minimal example that leads to the error:

d = {1:'a', 2:'b', 3:'c'}
print(d.keys()[0])

Output:

Traceback (most recent call last): File "C:\Users\...\code.py", line 2, in <module> print(d.keys()[0])
TypeError: 'dict_keys' object is not subscriptable

Note that the same error message occurs if you use slicing instead of indexing:

d = {1:'a', 2:'b', 3:'c'}
print(d.keys()[:-1]) # <== same error

Fixes

The reason this error occurs is that the dictionary.keys() method returns a dict_keys object that is not subscriptable.

You can use the type() function to check it for yourself:

print(type(d.keys()))
# <class 'dict_keys'>

Note: You cannot expect dictionary keys to be ordered, so using indexing on a non-ordered type wouldn’t make too much sense, would it?

You can fix the non-subscriptable TypeError by converting the non-indexable dict_keys object to an indexable container type such as a list in Python using the list() or tuple() function.

Here’s an example fix:

d = {1:'a', 2:'b', 3:'c'}
print(list(d.keys())[0])
# 1

Here’s an other example fix:

d = {1:'a', 2:'b', 3:'c'}
print(tuple(d.keys())[:-1])
# (1, 2)

Both lists and tuples are subscriptable so you can use indexing and slicing after converting the dict_keys object to a list or a tuple.

Full Guide: Python Fixing This Subsctiptable Error (General)

Summary

Python raises the TypeError: 'dict_keys' object is not subscriptable if you try to index x[i] or slice x[i:j] a dict_keys object.

The dict_keys type is not indexable, i.e., it doesn’t define the __getitem__() method. You can fix it by converting the dictionary keys to a list using the list() built-in function.

Alternatively, you can also fix this by removing the indexing or slicing call, or defining the __getitem__ method. Although the previous approach is often better.

What’s Next?

I hope you’d be able to fix the bug in your code! Before you go, check out our free Python cheat sheets that’ll teach you the basics in Python in minimal time:

If you struggle with indexing in Python, have a look at the following articles on the Finxter blog—especially the third!

Related Articles:

• Indexing in Python
• Slicing in Python
• Accessing the Index of Iterables in Python

Do you need to create a function that returns a NumPy array but you don’t know how? No worries, in sixty seconds, you’ll know! Go!

A Python function can return any object such as a NumPy Array. To return an array, first create the array object within the function body, assign it to a variable arr, and return it to the caller of the function using the keyword operation “return arr“.

Recommended Tutorial: How to Initialize a NumPy Array? 6 Easy Ways

Create and Return 1D Array

For example, the following code creates a function create_array() of numbers 0, 1, 2, …, 9 using the np.arange() function and returns the array to the caller of the function:

import numpy as np def create_array(): ''' Function to return array ''' return np.arange(10) numbers = create_array()
print(numbers)
# [0 1 2 3 4 5 6 7 8 9]

The np.arange([start,] stop[, step]) function creates a new NumPy array with evenly-spaced integers between start (inclusive) and stop (exclusive).

The step size defines the difference between subsequent values. For example, np.arange(1, 6, 2) creates the NumPy array [1, 3, 5].

To better understand the function, have a look at this video:

I also created this figure to demonstrate how NumPy’s arange() function works on three examples:

In the code example, we used np.arange(10) with default start=0 and step=1 only specifying the stop=10 argument.

If you need an even deeper understanding, I’d recommend you check out our full guide on the Finxter blog.

Recommended Tutorial: NumPy Arange Function — A Helpful Illustrated Guide

Create and Return 2D NumPy Array

You can also create a 2D (or multi-dimensional) array in a Python function by first creating a 2D or (xD) nested list and converting the nested list to a NumPy array by passing it into the np.array() function.

The following code snippet uses nested list comprehension to create a 2D NumPy array following a more complicated creation pattern:

import numpy as np def create_array(a,b): ''' Function to return array ''' lst = [[(i+j)**2 for i in range(a)] for j in range(b)] return np.array(lst) arr = create_array(4,3)
print(arr)

Output:

[[ 0 1 4 9] [ 1 4 9 16] [ 4 9 16 25]]

I definitely recommend reading the following tutorial to understand nested list comprehension in Python:

Recommended Tutorial: Nested List Comprehension in Python

More Ways

There are many other ways to return an array in Python.

For example, you can use either of those methods inside the function body to create and initialize a NumPy array:

• Method 1: Use np.array()
• Method 2: Use np.zeros()
• Method 3: Use np.ones()
• Method 4: Use np.full()
• Method 5: Use np.empty()
• Method 6: Use np.arange()
• Bonus: Initialize a NumPy array with CSV data

To get a quick overview what to put into the function and how these methods work, I’d recommend you check out our full tutorial.

Recommended Tutorial: How to Initialize a NumPy Array? 6 Easy Ways

Related Tutorials

• Python Return String From Function
• Python Return Dict From Function
• Python Return Set From Function

Programmer Humor

Q: How do you tell an introverted computer scientist from an extroverted computer scientist? A: An extroverted computer scientist looks at your shoes when he talks to you.

Question

Question: Can you use lists as values of a dictionary in Python?

This short article will answer your question. So, let’s get started right away with the answer:

Answer

You can use Python lists as dictionary values. In fact, you can use arbitrary Python objects as dictionary values and all hashable objects as dictionary keys. You can define a list [1, 2] as a dict value either with dict[key] = [1, 2] or with d = {key: [1, 2]}.

Here’s a concrete example showing how to create a dictionary friends where each dictionary value is in fact a list of friends:

friends = {'Alice': ['Bob', 'Carl'], 'Bob': ['Alice'], 'Carl': []} print('Alice friends: ', friends['Alice'])
# Alice friends: ['Bob', 'Carl'] print('Bob friends: ', friends['Bob'])
# Bob friends: ['Alice'] print('Carl friends: ', friends['Carl'])
# Carl friends: []

Note that you can also assign lists as values of specific keys by using the dictionary assignment operation like so:

friends = dict()
friends['Alice'] = ['Bob', 'Carl']
friends['Bob'] = ['Alice']
friends['Carl'] = [] print('Alice friends: ', friends['Alice'])
# Alice friends: ['Bob', 'Carl'] print('Bob friends: ', friends['Bob'])
# Bob friends: ['Alice'] print('Carl friends: ', friends['Carl'])
# Carl friends: []

Can I Use Lists as Dict Keys?

You cannot use lists as dictionary keys because lists are mutable and therefore not hashable. As dictionaries are built on hash tables, all keys must be hashable or Python raises an error message.

Here’s an example:

d = dict()
my_list = [1, 2, 3]
d[my_list] = 'abc'

This leads to the following error message:

Traceback (most recent call last): File "C:\Users\xcent\Desktop\code.py", line 3, in <module> d[my_list] = 'abc'
TypeError: unhashable type: 'list'

To fix this, convert the list to a Python tuple and use the Python tuple as a dictionary key. Python tuples are immutable and hashable and, therefore, can be used as set elements or dictionary keys.

Here’s the same example after converting the list to a tuple—it works!

d = dict()
my_list = [1, 2, 3]
my_tuple = tuple(my_list)
d[my_tuple] = 'abc'

Before you go, maybe you want to join our free email academy of ambitious learners like you? The goal is to become 1% better every single day (as a coder). We also have cheat sheets!

Do you need to create a function that returns a string but you don’t know how? No worries, in sixty seconds, you’ll know! Go!

A Python function can return any object such as a string. To return a string, create the string object within the function body, assign it to a variable my_string, and return it to the caller of the function using the keyword operation return my_string. Or simply create the string within the return expression like so: return "hello world"

def f(): return 'hello world' f()
# hello world

Create String in Function Body

Let’s have a look at another example:

The following code creates a function create_string() that iterates over all numbers 0, 1, 2, …, 9, appends them to the string my_string, and returns the string to the caller of the function:

def create_string(): ''' Function to return string ''' my_string = '' for i in range(10): my_string += str(i) return my_string s = create_string()
print(s)
# 0123456789

Note that you store the resulting string in the variable s. The local variable my_string that you created within the function body is only visible within the function but not outside of it.

So, if you try to access the name my_string, Python will raise a NameError:

>>> print(my_string)
Traceback (most recent call last): File "<pyshell#1>", line 1, in <module> print(my_string)
NameError: name 'my_string' is not defined

To fix this, simply assign the return value of the function — a string — to a new variable and access the content of this new variable:

>>> s = create_string()
>>> print(s)
0123456789

There are many other ways to return a string in Python.

Return String With List Comprehension

For example, you can use a list comprehension in combination with the string.join() method instead that is much more concise than the previous code—but creates the same string of digits:

def create_string(): ''' Function to return string ''' return ''.join([str(i) for i in range(10)]) s = create_string()
print(s)
# 0123456789

For a quick recap on list comprehension, feel free to scroll down to the end of this article.

You can also add some separator strings like so:

def create_string(): ''' Function to return string ''' return ' xxx '.join([str(i) for i in range(10)]) s = create_string()
print(s)
# 0 xxx 1 xxx 2 xxx 3 xxx 4 xxx 5 xxx 6 xxx 7 xxx 8 xxx 9

Return String with String Concatenation

You can also use a string concatenation and string multiplication statement to create a string dynamically and return it from a function.

Here’s an example of string multiplication:

def create_string(): ''' Function to return string ''' return 'ho' * 10 s = create_string()
print(s)
# hohohohohohohohohoho

String Concatenation of Function Arguments

Here’s an example of string concatenation that appends all arguments to a given string and returns the result from the function:

def create_string(a, b, c): ''' Function to return string ''' return 'My String: ' + a + b + c s = create_string('python ', 'is ', 'great')
print(s)
# My String: python is great

Concatenate Arbitrary String Arguments and Return String Result

You can also use dynamic argument lists to be able to add an arbitrary number of string arguments and concatenate all of them:

def create_string(*args): ''' Function to return string ''' return ' '.join(str(x) for x in args) print(create_string('python', 'is', 'great'))
# python is great print(create_string(42, 41, 40, 41, 42, 9999, 'hi'))
# 42 41 40 41 42 9999 hi

Background List Comprehension

Knowledge: List comprehension is a very useful Python feature that allows you to dynamically create a list by using the syntax [expression context]. You iterate over all elements in a given context “for i in range(10)“, and apply a certain expression, e.g., the identity expression i, before adding the resulting values to the newly-created list.

In case you need to learn more about list comprehension, feel free to check out my explainer video:

Programmer Humor

Q: How do you tell an introverted computer scientist from an extroverted computer scientist? A: An extroverted computer scientist looks at your shoes when he talks to you.