url_for Build Error

I'm new at flask and html. I get build error when I try to use action attribute in my html code and I don't have any idea why this happens. this is my html code:

<div class="post">
<form method=post action="{{url_for('add_post')}}" ></a>
<input type="text" name="post" placeholder="What's happenig?" /> 
<input type="submit" value="post" class="post_button" > 
</div><!--end of class post-->

and my flask:

@app.route('/mk/add_post/<username>' , methods=['POST'])
def add_post():
    if request.method == 'POST':
        text=request.form('post')
        user=session['username']        
        post(user , text)
        p='/mk/main/%s'%username
        return redirect(p)

and I get this error:

BuildError: ('add_post', {}, None)

any ideas?

Solution

you don't need to define your url path as @app.route('/mk/add_post/<username>' , methods=['POST']) as you are fetching the user name form session. This should work:

@app.route('/mk/add_post' , methods=['POST'])
def add_post():
    ...

In case you do wish to stick to '/mk/add_post/<username>'. Change your template to:

<form method=post action="{{url_for('add_post', username='mahnoosh')}}" ></a>

NOTE: Also you forgot to add username as argument.

@app.route('/mk/add_post/<username>' , methods=['POST'])
def add_post(username):
    ...

Here's how I tested:

python script:

from flask import *                                  

app = Flask(__name__)                                

@app.route('/mk/add_post/<username>' , methods=['POST'])       
def add_post(username):                              
    if request.method == 'POST':                     
        print username                               
        return username                              

@app.route('/')                                      
def main():                                          
    return render_template('so.html')                

if __name__ == "__main__":                           
    app.run(host='0.0.0.0', debug=True, port=5050)

so.html:

<html>                                                                        
<div class="post">                                                            
  <form method=post action="{{url_for('add_post', username='mahnoosh')}}" ></a>  
<input type="text" name="post" placeholder="What's happenig?" />              
<input type="submit" value="post" class="post_button" >                       
</div>                                                                        
</html>