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Selenium dioxide imparts a red colour to glass. It is used in small quantities to counteract the colour due to iron impurities and so to create (apparently) colourless glass. In larger quantities, it gives a deep ruby red colour. Selenium dioxide is the active ingredient in some cold-bluing solutions. Selenium PHP Selenium Python WebdriverIO4 WebdriverIO5 New. WebdriverIO6 Selenium IDE Cypress New. Playwright Testcafe New. Storybook CSF React. Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, and also has similarities to arsenic.

Adding the nutrient selenium to diets protects against obesity and provides metabolic benefits to mice, according to a study published today in eLife.

The results could lead to interventions that reproduce many of the anti-aging effects associated with dietary restriction while also allowing people to eat as normal.

Several types of diet have been shown to increase healthspan - that is, the period of healthy lifespan. One of the proven methods of increasing healthspan in many organisms, including non-human mammals, is to restrict dietary intake of an amino acid called methionine.

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Recent studies have suggested that the effects of methionine restriction on healthspan are likely to be conserved in humans. Although it might be feasible for some people to practice methionine restriction, for example, by adhering to a vegan diet, such a diet might not be practical or desirable for everyone. In the current study, a research team from the Orentreich Foundation for the Advancement of Science (OFAS), Cold Spring, New York, US, aimed to develop an intervention that produces the same effects as methionine restriction, while also allowing an individual to eat a normal, unrestricted diet.

An important clue for developing such a treatment is that methionine restriction causes a decrease in the amounts of an energy-regulating hormone called IGF-1. If a treatment could be found that causes a similar decrease in IGF-1, this might also have beneficial effects on healthspan. Previous research has shown that selenium supplementation reduces the levels of circulating IGF-1 in rats, suggesting that this could be an ideal candidate.

The team first studied whether selenium supplementation offered the same protection against obesity as methionine restriction. They fed young male and older female mice one of three high-fat diets: a control diet containing typical amounts of methionine, a methionine-restricted diet, and a diet containing typical amounts of methionine as well as a source of selenium. For both male and female mice of any age, the authors found that selenium supplementation completely protected against the dramatic weight gain and fat accumulation seen in mice fed the control diet, and to the same extent as restricting methionine.

Next, they explored the effects of the three diets on physiological changes normally associated with methionine restriction. To do this, they measured the amounts of four metabolic markers in blood samples from the previously treated mice. As hoped, they found dramatically reduced levels of IGF-1 in both male and female mice. They also saw reductions in the levels of the hormone leptin, which controls food intake and energy expenditure. Their results indicate that selenium supplementation produces most, if not all, of the hallmarks of methionine restriction, which suggests that this intervention may have a similar positive effect on healthspan.

To gain insight into the beneficial effects of selenium supplementation, the researchers used a different organism - yeast. The two most widely used measurements of healthspan in yeast are chronological lifespan, which tells us how long dormant yeast remain viable, and replicative lifespan, which measures the number of times a yeast cell can produce new offspring. The team previously showed that methionine restriction increases the chronological lifespan of yeast, so they tested whether selenium supplementation might do the same. As it turned out, yeast grown under selenium-supplemented conditions had a 62% longer chronological lifespan (from 13 days to 21 days) and a replicative lifespan extended by nine generations as compared with controls. This demonstrates that supplementing yeast with selenium produces benefits to healthspan detectable by multiple tests of cell aging.

'One of the major goals of aging research is to identify simple interventions that promote human healthspan,' notes senior author Jay Johnson, Senior Scientist at OFAS. 'Here we present evidence that short-term administration of either organic or inorganic sources of selenium provides multiple health benefits to mice, the most notable of which being the prevention of diet-induced obesity. In the long term, we expect that supplementation with these compounds will also prevent age-related disease and extend the overall survival of mice. It is our hope that many of the benefits observed for mice will also hold true for humans.'


This study will be included in eLife's Special Issue on Aging, Geroscience and Longevity. To view the Special Issue, visit

This tutorial will guide you through the concept of Selenium Webdriver waits (Implicit and Explicit Waits) and how to use them in Python.

If you are doing automation in Selenium Python and wish to write error-free scripts, then you must learn to use waits. They let you handle any unexpected condition which may occur.

While automating a web application, you intend to write a script which can execute the following tasks.

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  • Load up the browser,
  • Open up the website,
  • Locate the elements, and
  • Execute the needed action.

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However, the test script could sometimes fail due to the following reasons.

  • The web element was not present as it could not load due to the runtime issues.
  • The usage of AJAX techniques in the web applications has introduced uncertainty in the sense that loading of the web page and the web elements present in it may happen at a different time span.
  • Many times, due to peak traffic or network latency, our application may behave differently than it does at normal, optimal conditions.

In these cases, we need to wait for the time to allow the web elements to load completely, before using them in our tests. Webdriver API provides two types of wait mechanisms to handle such conditions. We will discuss these wait conditions one by one in detail.

Selenium Webdriver Waits in Python

The two types of Selenium Webdriver waits are :

  • Implicit Wait
  • Explicit Wait

Implicit Wait

An implicit wait directs the WebDriver to poll the DOM for a certain amount of time (as mentioned in the command) when trying to locate an element that is not visible immediately. The default value of time that can be set using Implicit wait is zero. Its unit is in seconds. Implicit wait remains associated with the web element until it gets destroyed.

Follow the below coding snippet illustrating the use of Implicit wait in Python.

In the above code, the implicitly_wait( ) method tells the Webdriver to poll the DOM again and again for a certain amount of time. The timeout in this example is 100 seconds which will trigger if the target element is not available during that period.

We define Implicit wait for a Webdriver object so it will remain active until the existence of that object.

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Explicit Wait

There may be scenarios when the wait time is uncertain. Explicit waits work as a savior there. Here we can define certain conditions, and Selenium WebDriver will proceed with the execution of the next step only after this condition gets fulfilled.

The explicit wait is the most preferred way of implementing Selenium webdriver waits in a test script. It provides the flexibility to wait for a custom condition to happen and then move to the next step.

Following are the two Selenium Python classes needed to implement explicit waits.

  • WebDriverWait, and
  • Expected Conditions class of the Python.

To understand its usage, let’s take an example of a Simple JavaScript alert on a webpage. A button is present on the web page to trigger that alert.

One of the use cases would be to click on the button and verify that it fires the alert on not. Using the Explicit wait with Expected conditions WebDriver will wait for the time only till the Simple JavaScript is not visible on the screen. As soon as it appears, the WebDriver will move on to execute the next step.

However, if the alert does not appear until the maximum wait time defined in the explicit wait, Webdriver will throw a “NoAlertPresentException.”

Here is the HTML code for generating a simple alert using JavaScript.

You need to save this file on your computer and name it as the ‘Simple_Alert.HTML.’

Following is the code snippet demonstrating the Explicit wait.

In the above script, the timeout value is 10 seconds. It means the Webdriver will wait for 10 seconds before throwing a TimeoutException.
If the element is present, then it may return within the 0 – 10 seconds. By default, the WebDriverWait API executes the ExpectedCondition every 500 milliseconds until it returns successfully.

If the ExpectedCondition matches, then the return value will be a Boolean (TRUE) whereas a non-null value for all other ExpectedCondition types.

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Standard Expected Conditions

There are a no. of standard conditions which you may commonly encounter while automating the web pages. Below is the list displaying the names of each of them. All of these classes are available in the “” Python module.

  1. class alert_is_present
    It allows waiting for an alert to appear.
  2. class element_located_selection_state_to_be(ui_locator, is_selected)
    It allows waiting for an element to locate having a specified state.
  3. class element_located_to_be_selected(ui_locator)
    It allows waiting for an element to locate in the selected state.
  4. class element_selection_state_to_be(ui_element, is_selected)
    It allows waiting to find an element having a specified state.
  5. class element_to_be_clickable(ui_locator)
    It allows waiting for an element to locate which is in a clickable state.
  6. class element_to_be_selected(ui_element)
    It allows waiting for an element to find which is in a selected state.
  7. class frame_to_be_available_and_switch_to_it(ui_locator)
    It allows waiting for a frame to become available for the switchover.
  8. class invisibility_of_element_located(ui_locator)
    It allows waiting for an element which is invisible or not in the DOM.
  9. class staleness_of(ui_element)
    It allows waiting for an element getting removed from the DOM.
  10. class text_to_be_present_in_element(ui_locator, inner_text)
    It allows waiting for an element to find with a given text.
  11. class text_to_be_present_in_element_value(ui_locator, value)
    It allows waiting for an element to find with a given text inside the locator.
  12. class title_contains(title_text)
    It allows waiting to find a title containing the specified text in a case-sensitive format.
  13. class title_is(title)
    It allows waiting to find a title matching the exact specified text.
  14. class visibility_of(ui_element)
    It allows waiting to find an element which is functionally visible.
  15. class visibility_of_all_elements_located(ui_locator)
    It allows waiting to find all functionally visible elements the specified locator will return.
  16. class visibility_of_any_elements_located(ui_locator)
    It allows waiting to find at least one functionally visible elements the specified locator will return.
  17. class visibility_of_element_located(ui_locator)
    It allows waiting to find a specific and functionally visible element the specified locator will return.

In Selenium Python binding, you can easily find methods to handle these. It saves you from writing any user-defined expected condition class or creating a package for the same.

Quick Wrapup – Selenium Webdriver Waits in Python

Understanding of Selenium Webdriver waits is a key to produce high-quality automation test scripts. We hope this tutorial would have directed you to the right approach.

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