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Chapter 6. Variables, Assignment and Input |
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The basic assignment statement does more than assign the result of a single expression to a single variable. The assignment satement also copes nicely with assigning multiple variables at one time. The left and right side must have the same number of elements. For example, the following script has several examples of multiple assignment.
Example 6.7. line.py
When we run this program, we get the following output
We set variables x1 , y1 , x2 and y2 . Then we computed m and b from those four variables. Then we printed the m and b .
The basic rule is that Python evaluates the entire right-hand side of the = statement. Then it matches values with destinations on the left-hand side. If the lists are different lengths, an exception is raised and the program stops.
Because of the complete evaluation of the right-hand side, the following construct works nicely to swap to variables. This is often quite a bit more complicated in other languages.
We'll return to this in Chapter 13, Tuples , where we'll see additional uses for this feature.
Input Functions | The Statement |
Published under the terms of the Open Publication License |
Think of a variable as a name attached to a particular object . In Python, variables need not be declared or defined in advance, as is the case in many other programming languages. To create a variable, you just assign it a value and then start using it. Assignment is done with a single equals sign ( = ).
00:00 Welcome to this first section, where we’ll talk about variable assignments in Python. First of all, I just want to mention that I’m going to be using the IPython shell.
00:09 The reason for that is just that it adds a bit of colors to the prompt and makes it a bit easier to see which types we’re working with, but you don’t need to do this install that I’m going to show you in just a second.
00:19 You can just use your normal Python interpreter and it’s going to work all the same. If you want to install IPython, all you need to do is go to your terminal and type pip3 install ipython , press Enter, and then wait until it installs.
00:36 I already got it installed. And then instead of typing python to get into the interpreter, you’re going to type ipython .
00:46 It gives us the same functionality, only you see there’s some colors involved and it looks a bit nicer. I can do clear and clear my screen. So it’s going to make it a bit easier for you to follow, but that’s all.
00:58 So, first stop: a standard variable assignment in Python. Unlike other languages, in Python this is very simple. We don’t need to declare a variable, all we need to do is give it a name, put the equal sign ( = ) and then the value that we want to assign. That’s it.
01:15 That’s a variable assignment in Python. I just assigned the value 300 to the variable n . So now I can print(n) and get the result.
01:26 Or, since I’m in an interpreter session, I can just put in n and it shows me that the output is going to be 300 . So, that’s the basic, standard variable assignment that you’re going to do many times in Python.
01:38 And it’s nice that you don’t need to declare the variable before. You simply can type it in like this. Now the variable n is referring to the value 300 .
01:48 What happens if I change it? So, I don’t need to stick with 300 through the lifetime of this variable. I can just change it to something else. I can say “Now this is this going to be 400 .”
02:00 Or, in Python, not even the type is fixed, so I can say n = "hello" and change it to a string.
02:10 And this is still all working fine. So you see, it feels very fluid, and this is because Python is a dynamically-typed language, so we don’t need to define types and define variables beforehand and then they’re unchangeable for the rest of the program—but it’s fluid, and we can just start off with n being an integer of the value of 300 and through the lifetime of the program, it can take on a couple of different identities.
02:36 So, apart from the standard variable assignment that we just saw before, n = 300 , we can also use a chained assignment in Python, which makes it quick to assign a couple of variables to the same value.
02:49 And that looks like this.
02:52 I can say n = m = x = y et cetera, and then give it a value. And now all of those variable names point to 400 , so I can say m is 400 , x is 400 , y is 400 , et cetera. That’s what is called a chained assignment in Python.
03:15 Another way is the multiple assignment statement, or multiple assignments, which works a little bit different and there’s something you need to take care of, but I still want to introduce you to it. If you go ahead here, I can assign two values at the same time in one line.
03:32 So I can say a, b = 300, 400 . The comma ( , ) is important, and it’s important that the amount of variables that you’re declaring here on the left side is the same amount of values that you have on the right side.
03:48 I can do this, and now b points to 400 , a points to 300 .
03:54 It doesn’t have to be two, there can be more, but just make sure that every time if you use this multiple assignment statement, that the amount of variables you use left is the same as the amount of values on the right. And as a last point in this section, I want to talk a little bit about variable types.
04:14 I already mentioned that variable types don’t have to be fixed in Python. I can start off with
04:21 n pointing to 300 , which as we know is an integer. Remember, you can always check what the type of a variable is by just saying type() and passing in the variable in there.
04:33 So it gives me as an output that this is an int (integer).
04:37 This is just the same as saying “What’s the type() of 300 or 200 ?” directly— it’s an integer—because all that I’m passing in here is a reference to this object. We’ll talk about this more in the next section.
04:52 But now I can easily change the type of this variable, because all I’m doing is pointing it to a different object. So now n is pointing to a string.
05:01 If I say type(n) now, it will tell me it’s a str (string).
05:08 And the reason for this is that variables in Python are simply references to objects. In the next section, we’ll talk much more what’s important about that and how in Python everything is an object.
05:19 And that it for this section! Let’s do a quick recap. Variable assignments in Python are pretty straightforward. We have the standard variable assignment that just goes <variable name> = <your value> .
05:32 We have another way of doing it that’s called chained assignments, where we can assign a couple of variable names to the same value by just using multiple equal signs.
05:43 Then there’s the multiple assignment statement, which works a little differently, and you have to take care to use commas and the same amount of variable names on the left side as values on the right side.
05:53 It’s going to assign, as expected, n to 300 , m to 400 . And then finally, we talked about variable types, that they are fluid in Python and that you can check what the variable type is by using the type() function.
06:07 And here’s a nice thing to see also, that n is just a pointer to the 300 integer, because we’re going to get the same result if we say type(n) or type(300) .
06:18 They’re both int (integer) objects. And this is a concept that we’re going to talk about more in the upcoming section when we talk about object references. See you over there.
iamscottdavis on Dec. 10, 2019
I installed ipython on my chromebook but it won’t run.
Martin Breuss RP Team on Dec. 10, 2019
You might have to close and re-open your terminal @iamscottdavis
Geir Arne Hjelle RP Team on Dec. 10, 2019
I’ve recently had some weird issues with prompt_toolkit , one of the dependencies of IPython. Maybe that’s what you’re running into?
I got a cryptic error message like TypeError: __init__() got an unexpected keyword argument 'inputhook' . If this is your problem as well, the best solution should be to update to IPython >= 7.10 which should have fixed this. Another workaround is to downgrade prompt_toolkit to version 2.
See some discussion on github.com/ipython/ipython/issues/11975
If you are having other problems, feel free to post your error messages :)
kiran on July 18, 2020
if i declare the variable in any one loop in python.
now my question is a is local/global variable? in C it is local variable but what about python? in Python even i declare a variable with in the loop it become a global variable?
Martin Breuss RP Team on July 18, 2020
In Python, it will keep the last value it got assigned within the loop also outside in the global scope. That is why a is still accessible and has a value in your example, also outside of the loop’s scope.
DoubleA on Jan. 20, 2021
Hey Martin,
Thanks for pulling this stuff together and explaining it so clearly. I came accross some sort of a variation of the multiple assignment you discussed. Basically, it seems that the number of variable names and variables can be diffrent. What I mean is this:
then print(a,b,c) gives me the following output:
Am I right saying that, basically, what happens above is that the variable c having an asterisk before it will get assigned a list of the two values, incl. the excessive (‘string’) one?
Referring back to Kiran’s question:
When I run this code:
I see that the globals() function returns, amongst other things, the value of the variables a , b and the last value of the iterable elem . Both variables a and b appear to be visible in the global scope as key-value pairs of the following dict:
Bartosz Zaczyński RP Team on Jan. 21, 2021
@DoubleA The “starred” expression syntax you were referring to before can be used for extended iterable unpacking .
Become a Member to join the conversation.
Assignment operators in python.
The Python Operators are used to perform operations on values and variables. These are the special symbols that carry out arithmetic, logical, and bitwise computations. The value the operator operates on is known as the Operand. Here, we will cover Different Assignment operators in Python .
Operators |
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| = | Assign the value of the right side of the expression to the left side operand | c = a + b |
| += | Add right side operand with left side operand and then assign the result to left operand | a += b |
| -= | Subtract right side operand from left side operand and then assign the result to left operand | a -= b |
| *= | Multiply right operand with left operand and then assign the result to the left operand | a *= b |
| /= | Divide left operand with right operand and then assign the result to the left operand | a /= b |
| %= | Divides the left operand with the right operand and then assign the remainder to the left operand | a %= b |
| //= | Divide left operand with right operand and then assign the value(floor) to left operand | a //= b |
| **= | Calculate exponent(raise power) value using operands and then assign the result to left operand | a **= b |
| &= | Performs Bitwise AND on operands and assign the result to left operand | a &= b |
| |= | Performs Bitwise OR on operands and assign the value to left operand | a |= b |
| ^= | Performs Bitwise XOR on operands and assign the value to left operand | a ^= b |
| >>= | Performs Bitwise right shift on operands and assign the result to left operand | a >>= b |
| <<= | Performs Bitwise left shift on operands and assign the result to left operand | a <<= b |
| := | Assign a value to a variable within an expression | a := exp |
Here are the Assignment Operators in Python with examples.
Assignment Operators are used to assign values to variables. This operator is used to assign the value of the right side of the expression to the left side operand.
The Addition Assignment Operator is used to add the right-hand side operand with the left-hand side operand and then assigning the result to the left operand.
Example: In this code we have two variables ‘a’ and ‘b’ and assigned them with some integer value. Then we have used the addition assignment operator which will first perform the addition operation and then assign the result to the variable on the left-hand side.
The Subtraction Assignment Operator is used to subtract the right-hand side operand from the left-hand side operand and then assigning the result to the left-hand side operand.
Example: In this code we have two variables ‘a’ and ‘b’ and assigned them with some integer value. Then we have used the subtraction assignment operator which will first perform the subtraction operation and then assign the result to the variable on the left-hand side.
The Multiplication Assignment Operator is used to multiply the right-hand side operand with the left-hand side operand and then assigning the result to the left-hand side operand.
Example: In this code we have two variables ‘a’ and ‘b’ and assigned them with some integer value. Then we have used the multiplication assignment operator which will first perform the multiplication operation and then assign the result to the variable on the left-hand side.
The Division Assignment Operator is used to divide the left-hand side operand with the right-hand side operand and then assigning the result to the left operand.
Example: In this code we have two variables ‘a’ and ‘b’ and assigned them with some integer value. Then we have used the division assignment operator which will first perform the division operation and then assign the result to the variable on the left-hand side.
The Modulus Assignment Operator is used to take the modulus, that is, it first divides the operands and then takes the remainder and assigns it to the left operand.
Example: In this code we have two variables ‘a’ and ‘b’ and assigned them with some integer value. Then we have used the modulus assignment operator which will first perform the modulus operation and then assign the result to the variable on the left-hand side.
The Floor Division Assignment Operator is used to divide the left operand with the right operand and then assigs the result(floor value) to the left operand.
Example: In this code we have two variables ‘a’ and ‘b’ and assigned them with some integer value. Then we have used the floor division assignment operator which will first perform the floor division operation and then assign the result to the variable on the left-hand side.
The Exponentiation Assignment Operator is used to calculate the exponent(raise power) value using operands and then assigning the result to the left operand.
Example: In this code we have two variables ‘a’ and ‘b’ and assigned them with some integer value. Then we have used the exponentiation assignment operator which will first perform exponent operation and then assign the result to the variable on the left-hand side.
The Bitwise AND Assignment Operator is used to perform Bitwise AND operation on both operands and then assigning the result to the left operand.
Example: In this code we have two variables ‘a’ and ‘b’ and assigned them with some integer value. Then we have used the bitwise AND assignment operator which will first perform Bitwise AND operation and then assign the result to the variable on the left-hand side.
The Bitwise OR Assignment Operator is used to perform Bitwise OR operation on the operands and then assigning result to the left operand.
Example: In this code we have two variables ‘a’ and ‘b’ and assigned them with some integer value. Then we have used the bitwise OR assignment operator which will first perform bitwise OR operation and then assign the result to the variable on the left-hand side.
The Bitwise XOR Assignment Operator is used to perform Bitwise XOR operation on the operands and then assigning result to the left operand.
Example: In this code we have two variables ‘a’ and ‘b’ and assigned them with some integer value. Then we have used the bitwise XOR assignment operator which will first perform bitwise XOR operation and then assign the result to the variable on the left-hand side.
The Bitwise Right Shift Assignment Operator is used to perform Bitwise Right Shift Operation on the operands and then assign result to the left operand.
Example: In this code we have two variables ‘a’ and ‘b’ and assigned them with some integer value. Then we have used the bitwise right shift assignment operator which will first perform bitwise right shift operation and then assign the result to the variable on the left-hand side.
The Bitwise Left Shift Assignment Operator is used to perform Bitwise Left Shift Opertator on the operands and then assign result to the left operand.
Example: In this code we have two variables ‘a’ and ‘b’ and assigned them with some integer value. Then we have used the bitwise left shift assignment operator which will first perform bitwise left shift operation and then assign the result to the variable on the left-hand side.
The Walrus Operator in Python is a new assignment operator which is introduced in Python version 3.8 and higher. This operator is used to assign a value to a variable within an expression.
Example: In this code, we have a Python list of integers. We have used Python Walrus assignment operator within the Python while loop . The operator will solve the expression on the right-hand side and assign the value to the left-hand side operand ‘x’ and then execute the remaining code.
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We all know multiple assignment can assign multiple variables at one time , and it is useful in swap. It works well in this situation:
, but it failed in more complex situation, such as:
It seems in nums[nums[0]] , nums[0] will be assigned before , not at one time. it also failed in complex linklist node swap, such as:
So I want to know the mechanism behind multiple assignment in Python, and what is the Best Practices for this, temporary variable is the only way?
is equivalent to
Personally I would recommend to write complex assignments explicitly with temporary variables as you showed it.
Another way is to carefully choose the order of the elements in the assignment:
Changes nums as you expect.
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All credit goes to @MarkDickinson, who answered this in a comment: Notice the + in (target_list "=")+, which means one or more copies.In foo = bar = 5, there are two (target_list "=") productions, and the expression_list part is just 5. All target_list productions (i.e. things that look like foo =) in an assignment statement get assigned, from left to right, to the expression_list on the right ...
Here, variable represents a generic Python variable, while expression represents any Python object that you can provide as a concrete value—also known as a literal—or an expression that evaluates to a value. To execute an assignment statement like the above, Python runs the following steps: Evaluate the right-hand expression to produce a concrete value or object.
Python assigns values from right to left. When assigning multiple variables in a single line, different variable names are provided to the left of the assignment operator separated by a comma. The same goes for their respective values except they should be to the right of the assignment operator. While declaring variables in this fashion one ...
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With the exception of assigning to tuples and multiple targets in a single statement, the assignment done by augmented assignment statements is handled the same way as normal assignments. Similarly, with the exception of the possible in-place behavior, the binary operation performed by augmented assignment is the same as the normal binary ...
The multiple assignment syntax, often referred to as tuple unpacking or extended unpacking, is a powerful feature in Python. There are several ways to assign multiple values to variables at once. Let's start with a first example that uses extended unpacking. This syntax is used to assign values from an iterable (in this case, a string) to ...
Mastering Multiple Variable Assignment in Python. Python's ability to assign multiple variables in a single line is a feature that exemplifies the language's emphasis on readability and efficiency. In this detailed blog post, we'll explore the nuances of assigning multiple variables in Python, a technique that not only simplifies code but also ...
In this video, we will explore how to assign multiple variables in one line in Python. This technique allows for concise and readable code, especially when you need to initialize multiple variables simultaneously. This tutorial is perfect for students, professionals, or anyone interested in enhancing their Python programming skills.
Multiple- target assignment: x = y = 75. print(x, y) In this form, Python assigns a reference to the same object (the object which is rightmost) to all the target on the left. OUTPUT. 75 75. 7. Augmented assignment : The augmented assignment is a shorthand assignment that combines an expression and an assignment.
Learn the basics of assignment statements in Python in this tutorial. We'll cover the syntax and usage of the assignment operator, including multiple assignm...
In Python, you can use multiple assignments to assign values to multiple variables in a single line. This can make your code more concise and readable. Multiple Assignments. Python allows us to assign the same value to multiple variables. For Example. Consider the following statement: a=b=c=5
Conversely, assignment expressions don't support the advanced features found in assignment statements: Multiple targets are not directly supported: x = y = z = 0 # Equivalent: (z := (y := (x := 0))) ... rather than a statement as is Python's way. This allows assignments in more contexts, including contexts where comparisons are more common.
Assigning multiple variables in one statement. In an assignment using the ' = ' operator the right hand side expression is evaluated first. This provides a convenient way to swap the values of two variables using tuples: a, b = b, a. Here, the righthand side is packed into a tuple object, which is then unpacked into the variables assigned on ...
Python Multiple Assignment Statements In One Line. 1. A pythonic (3) way to declare more than one variable on a line. 13. How Python assign multiple variables at one line works? 1. How to assign 2 variables with the same parameters in 1 line. 0. Python Variable Assignment on One line. 0.
The basic assignment statement does more than assign the result of a single expression to a single variable. The assignment satement also copes nicely with assigning multiple variables at one time. The left and right side must have the same number of elements. For example, the following script has several examples of multiple assignment.
5. Multiple assignment evaluates the values of everything on the right hand side before changing any of the values of the left hand side. In other words, the difference is this: a = 1. b = 2. a = b # a = 2. b = a + b # b = 2 + 2. vs. this:
Variable Assignment. Think of a variable as a name attached to a particular object. In Python, variables need not be declared or defined in advance, as is the case in many other programming languages. To create a variable, you just assign it a value and then start using it. Assignment is done with a single equals sign ( = ).
Assignment statements support multiple targets: >>> age = 10 >>> st, reject = ('Kid', True) if age < 10 else ('Adult', False) >>> st, reject ('Adult', False) Share. ... Ternary Operator Python single variable assignment python 3.8. Hot Network Questions Professor Previously wrote letter, and now he's not responding to my emails ...
The Walrus Operator in Python is a new assignment operator which is introduced in Python version 3.8 and higher. This operator is used to assign a value to a variable within an expression. Syntax: a := expression. Example: In this code, we have a Python list of integers. We have used Python Walrus assignment operator within the Python while loop.
So I want to know the mechanism behind multiple assignment in Python, and what is the Best Practices for this, temporary variable is the only way? python; multiple-assignment; parallel-assignment; Share. Improve this question. Follow edited Nov 7, 2023 at 23:47. Karl Knechtel ...