Lists

How do read the contents of a file in Python and insert these lines into a list? Using the built-in function open() and the asterisk operator * a file’s contents can easily be translated into a list with the following one-liner: [*open('my_file.txt')]. What Does open() Function Do? The built-in open() function has one required parameter and several optional parameters. The required parameter is the location of the file. If the file is located in the current folder where the Python script is run you can insert the name of the file, for example, test.txt. ...

How do you prepend a value or item to a list? Or how do you prepend the contents of a list to another list using Python? There are two ways to prepend items to an existing list in Python: for single items use the list method .insert(idx, value) – this method will mutate the original list, or use the list concatenation operator new_list + old_list which can be set to a new variable and prevent the original list being mutated. ...

What does [:-1] in Python actually do and why would you want to use it? [:-1] in Python is a slice operation used on strings or lists and captures all contents of the string or list except for the last character or element. Here are some examples demonstrating the operation of this code with strings: 1 2 3 >>> my_string = "Why?" >>> my_string[:-1] 'Why' As you can see from the simple code example above a variable my_string contains a string that ends with a question mark. By using the [:-1] slice operation it outputs the whole string except for the last character in the string. ...

What does the asterisk before a variable name mean in Python? For example, if I have a variable that contains a list of strings and the name of the variable is called my_list what does *my_list do? The asterisk is an operator in Python that is commonly known as the multiplication symbol when used between two numbers (2 * 3 will produce 6) but when it is inserted at the beginning of a variable, such as an iterable, like a list or dictionary, it expands the contents of that variable. ...

How do you zip two lists together in Python? What if you want to zip multiple lists together, is it the same process or something different? The built-in function zip() allows users to combine iterables, such as a list, into tuples by taking each corresponding item from the lists passed into its parameters merging the lists into one. It does not matter whether you have two or multiple lists the function are the list parameters are the same. ...

How do you remove duplicates from a list using Python? To remove duplicate elements from a list in Python use a list comprehension to create a new list with no duplicates with this code: [x for idx, x in enumerate(original_list) if x not in original_list[idx+1:]] Here is an example demonstrating how this code works: 1 2 3 4 >>> original_list = [1, 2, 1, 3, 1, 2, 3] >>> [x for idx, x in enumerate(original_list) if x not in original_list[idx+1:]] [1, 2, 3] As you can see from the example above the result from the list comprehension produces a unique list from the original. ...

The slice operator enables you to capture a subset of data from an original list or string using the format [start:stop:step]. Some popular use cases where I have used the slice operator include: Extracting year and/or month and/or day of the month from a string Extracting the zip code at the tail end of an address string Masking bank account or credit card numbers Extracting area code from phone numbers Here is how the Python [:] operator can work when populating the values on either side of the colon: ...

What is the syntax for writing a for loop on one line in Python? This syntax is known as a list comprehension and enables the user to write a for loop on one lin To write a for loop on one line in Python, known more commonly as the list comprehension, wrap the for loop in a list like so: [elem for elem in my_loop]. Here is an example demonstrating how this code works: ...

Creating a list in Python can be super easy, but how do you populate that list with a bunch on zeroes? To create a list of zeroes in Python use the syntax [0] * n where n represents the number of items needed to create the list of zeroes required. Here’s an example demonstrating this use: 1 2 3 >>> my_zeros_list = [0] * 5 >>> print(my_zeros_list) [0, 0, 0, 0, 0] As you can see from the above example the list containing five elements are all populated with the value of 0. ...

How do you know if an element in Python is empty? First, you need to define what is meant by the term empty. Does it mean None or an empty string "" or something else? For the examples in this article, I’ll assume that empty means an element is defined as None, however, if you’ve defined empty as something else then you can simply substitute my None references to your definition of an empty element. ...