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Solve the following problem using Python, implementing the functions described below, one line at a time
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<USER_TASK:>
Description:
 def render_alert(content, alert_type=None, dismissable=True):
 """ Render a Bootstrap alert """

    button = ""
    if not alert_type:
        alert_type = "info"
    css_classes = ["alert", "alert-" + text_value(alert_type)]
    if dismissable:
        css_classes.append("alert-dismissable")
        button = (
            '<button type="button" class="close" '
            + 'data-dismiss="alert" aria-hidden="true">&times;</button>'
        )
    button_placeholder = "__BUTTON__"
    return mark_safe(
        render_tag(
            "div",
            attrs={"class": " ".join(css_classes)},
            content=button_placeholder + text_value(content),
        ).replace(button_placeholder, button)
    )